Envestigasyon Resous Dlo: Rejyon Metwopolitèn Pòtoprens, Repiblik Ayiti

Water Resource Investigations Port - au - Prince Metropoli tan Region R epubl ic of Haiti Author s : James K. Adamson Javan Miner Pierre - Yves Rochat Edito rs: S ergio Perez Monforte Maria Rod riguez TECHNICAL NOTE N o IDB - TN - 2446 M arch 2022 Water and Sanitation Division Water Resource Investigations Port - au - Prince Metropoli tan Region R epubl ic of Haiti Author s: James K. Adamson Javan Miner Pierre - Yves Rochat Edito rs: S ergio Perez Monforte Maria Rod riguez Inter - American Development Bank Water and Sanitation Division M arch 2022 Cataloging - in - Publication data provided by the Inter - American Development Bank Felipe Herrera Library Adamson, James K. Water resource investigations : Port - au - Prince metropolitan region, Republic of Haiti / James K. Adamson, Javan Miner, Pierre - Yves Rochat; Sergio Pérez Monforte, María Rodríguez. p. cm. — (IDB Technical Note; 2446) Includes bibliographic references. 1. Wa ter resources development - Haiti. 2. Watershed management - Haiti. 3. Water supply - Environmental aspects - Haiti. I. Miner, Javan. II. Rochat, Pierre - Yves. III. Pérez Monforte, Sergio, editor. IV. Rodríguez, María, editor. V. Inter - American Development B ank. Water and Sanitation Division. VI. Title. VII. Series. IDB - TN - 2446 Keywords: Water resource, hydrology, source monitoring, source protection, groundwater flow model JEL C odes: L95, Q25 http://www.iadb.org Copyright © [ 2022 ] Inter - American Development Bank. This work is licensed under a Creative Commons IGO 3.0 Attribution - NonCommercial - NoDerivatives (CC - IGO BY - NC - ND 3.0 IGO) license ( http://creativecommons.org/licenses/by - nc - nd/3.0/igo/legalcode ) and may be reproduced with attribution to the IDB and for any non - commercial purpose. No derivative work is allowed. Any dispute related to the use of the works of the IDB that cannot be settled amicably shall be submitted to arbitra tion pursuant to the UNCITRAL rules. 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WATER RESOURCE INVESTIGATIONS Port-au-Prince Metropolitan Region Republic of Haiti AUTHORS: James K. Adamson, PG Javan Miner, EIT Pierre-Yves Rochat EDITORS: Sergio Perez Monforte Maria Rodriguez March 2022 1 WATER RESOURCE INVESTIGATIONS Port-au-Prince Metropolitan Region Republic of Haiti With support from: Northwater International & Rezodlo, S.A, DINEPA, OREPA-Ouest and CTE-RMPP 2 Table of Contents PREFACE AND SUMMARIES Hydrogeological Investigation of Tunnel Diquini: Characterization of Hydrology and Guidance for Source Monitoring and Protection. Hydrogeological Investigation of Source Mariani: Characterization of Hydrology and Guidance for Source Monitoring and Protection. Plaine du Cul-de-Sac Groundwater Flow Model. Water Resource Investigations - Study Area. 3 Preface and Summaries This work is financed with support from the AquaFund. The AquaFund is the IDB’s thematic fund for water and sanitation, and has been the main financing mechanism to support the Bank’s investments in the sector since its creation in 2008. The AquaFund has contributed to the achievement of the Millennium Development Goals for water and sanitation in Latin America and the Caribbean and will play a crucial role in supporting the region’s governments in achieving the new Sustainable Development Goals. It has done so by facilitating investments to increase the provision of water and sanitation, water resources management, solid waste management, and wastewater treatment, while contributing to the sustainability and accessibility of these services for low- income populations. It also supports the Bank’s client countries in addressing the emerging challenges of climate change, rapid degradation of freshwater ecosystems, and increasing water insecurity. The AquaFund is financed through the IDB’s own resources and resources from donor partners, namely the Government of Austria, the Spanish Agency for International Development Cooperation AECID, the PepsiCo Foundation, and the Swiss Agency for Development and Cooperation SDC and the State Secretariat for Economic Affairs SECO. Background and context . The metropolitan area of Port-au-Prince has an estimated population of 2.8 million and projected to increase to 3.5 million by 2030 (CIA 2020). With an estimated water demand of 365,000 m³/day, protecting existing supplies and securing additional water supply is an urgent priority for the region. Since the 1980s, there has been insufficient investment in advancing knowledge of the primary resources that Port-au-Prince greatly relies on. The Port-au-Prince water supplies include: • Massif de la Selle aquifer system, a karst carbonate bedrock aquifer system that feeds fifteen springs and one well that supply the network. The average supply from these sources is approximately 120,000 m³/day. • The Plaine du Cul-de-Sac aquifer is an unconsolidated alluvial aquifer. There are 26 municipal wells that take ~37,000 m³/ day based on 2015-2018 data. Municipal pumping can potentially be increased by ~45,000 m³/day through new proposed wells and rehabilitation of inactive wells. Based on this information, there is a significant gap between water supply and demand. This has prompted the urgent need to protect existing resources and secure additional water supplies. In 2018 – 2019, Northwater International and Rezodlo SA were contracted to perform three water resource investigations in the Port- au-Prince region. The Government of Haiti supported the research both in the field and with historical data compilation. The investigations included: 1. Characterization of Tunnel Diquini 2. Characterization of Source Mariani 3. Groundwater Flow Model for the Plaine du Cul-de-Sac aquifer Tunnel Diquini is a ~1.5 km long tunnel constructed in 1940 that collects groundwater from fractures and a fault in the Massif de la Selle carbonate aquifer system. It is the single largest water source for the Port-au- Prince municipal water system, accounting for approximately 26% of the supply. Source Mariani is the second largest naturally flowing source that serves the Port-au-Prince municipal water system. The spring is a primary discharge of the Massif de la Selle carbonate aquifer system, the same aquifer complex as Tunnel Diquini. Due to its distal location from Port-au-Prince and low elevation, it requires a pumping system. When in operation, the spring accounts for approximately 17% of the Port-au-Prince water supply. The Plaine du Cul-de-Sac aquifer is one of Haiti’s most productive and largest aquifers (~360 km² aerial extent). This multi-layer unconsolidated alluvial aquifer has a thickness of more than 200 m, and supports municipal, 4 private and agricultural wells. The Port- au-Prince municipal system includes ~ 26 wells that serve the lower-lying areas of the metropolitan region. Historical peak aquifer- wide abstraction was estimated to be nearly 300,000 m³/day in the 1970s and 1980s as a result of the sugar cane industry, thus suffering declines in the water tables and increases in salinity. Current abstraction is significantly lower than historical peaks due to unreliable power and lack of commercial agriculture. A groundwater flow modeling exercise was performed to establish a preliminary estimate of renewable quantities of groundwater, and to improve the understanding of recharge origins and the interaction between groundwater and surface water. These parameters are important to guide water supply planning and development for Port-au-Prince and, at the same time, to manage and protect these resources. The objective of this research is to improve the understanding of Port-au-Prince’s critical water resources and to guide informed planning and investments to secure sustainable water supplies to satisfy the growing demand from the region. These studies guide further research and reveal important monitoring needs to strengthen resource characterization and data-driven decision-making and resource management. These investigations are preliminary in nature and limited due to the scarcity of data. I. Tunnel Diquini Summary Tunnel Diquini is the largest single source of water for the municipal water system of Port-au-Prince. Based on records from 2014-2018, the tunnel supplies an average of 29,449 m³/day to the metropolitan Port-au- Prince water system. The tunnel accounts for ~26% of total municipal production, and 37% of all gravity-fed spring flow that the metropolitan area obtains. An inspection was performed to characterize the hydrology of the tunnel waters and to better understand the origin of its flow and its relationship with groundwater and surface water systems. This knowledge is important to guide future studies and monitoring of the tunnel and to aid the Centre Technique d’Exploitation de la Région Métropolitaine de Port-Au-Prince (CTE-RMPP) in water use planning and management. The investigation was accomplished by using a combination of literature review, satellite and topographic imagery analysis, and field reconnaissance. A brief field mission to the tunnel was conducted on April 15, 2018, including: (i) physical and chemical sampling, (ii) stable isotope sampling, (iii) chlorofluorocarbon (CFC) and sulfur hexafluoride (SF6) sampling, (iv) flow rate measurement and (v) visual inspection of tunnel geology. Note: Additional data collection and research since the Tunnel Diquini study was conducted warrants revisiting and updating some of the interrogations and findings of the report. This summary presents data primarily based on the original report, with the exception of an increased range of recharge rate and a decreased catchment area based on updated research. Photo 1. Limestone exposed in the main tunnel. Photo 2. The tunnel portal facing into the tunnel. 5 Flow Characteristics 1. Recharge to the Massif de la Selle carbonate aquifer system and tunnel appears to be largely affected by high intensity and high-volume rainfall events such as hurricanes and tropical storms. It appears to be a 3-to-7-year cycle of recharge trends partially influenced by El Niño and La Niña events. 2. Tunnel discharge varies seasonally with recorded flows ranging from 11,085 to 73,265 m³/d, with a geometric mean for all known recorded flows of 27,987 m³/d (1980-2018 dataset). 3. Decreases in tunnel flow result from extended periods of normal precipitation and consecutive years without high intensity rainfall periods such as tropical storms and hurricanes. a. The recharge dynamics and flow regression can influence flow trends over periods of several years. Tunnel flows do not appear to have been decreasing over the long-term. b. Limited historical data from 1959 suggests that dry season low-flow conditions were comparable or perhaps lower than current low-flow conditions and strongly influenced by major recharge and drought events. c. The response time of the aquifer to major recharge events such as hurricanes may be shortening, possibly due to land cover and climatic changes. Figure 1. Conceptual cross section of Diquini Tunnel groundwater flow. 6 Groundwater / Surface Water Interaction 1. Based on water chemistry and tracers, the tunnel and Riviere Froide are connected to the same regional karst limestone aquifer and gain flow from it. The Riviere Froide may recharge the aquifer at various spatial and temporal extents, and this could result in a possible link between the tunnel and river system. 2. Based on geology and structure, Tunnel Diquini does not appear to have a hydraulic connection to the Riviere Momance, which is located farther to the south in the mountains and flows westerly to the Plaine de Leogane. a. The EPG fault zone and a perpendicular fault appear to direct groundwater in the Momance basin either into the Riviere Momance or into the lower reaches of the Riviere Froide, below where recharge to the tunnel would likely occur. Spatial Distribution of Groundwater Recharge 1. The long-term mean annual recharge rate in the karst terrain is estimated greater than 40% of annual precipitation. Recharge rates can be higher in years with tropical storms and hurricanes, and less than 15% in years with normal or low precipitation. 2. Recharge rates are higher during high intensity precipitation events, and lower during periods of average and low precipitation. 3. Aquifer storage of the ‘spring shed’ of the tunnel is estimated between 265 and 327 Mm³. 4. The aquifer is well mixed and has an average groundwater age of 26 to 32 years based on a single sampling event. 5. Recharge area of the tunnel flow appears to in the range of 12.4 km². a. Recharge area is dependent on the rate and duration of Riviere Froide leakage to the regional carbonate aquifer. b. The average recharge elevation is estimated at 650m, indicating that a portion of tunnel flow may originate from river leakage from the Riviere Froide to the regional carbonate aquifer. 7 Aquifer Vulnerability 1. Due to the high permeability and high infiltration rates typical in karst limestone environments, the tunnel waters are vulnerable to contamination. For example, fecal coliform, E. coli, and salmonella contamination was reported by Eptisa in March 2014. 2. Urbanization and land use changes in the hills south of the tunnel portal may have negative impacts to tunnel water quality and flow. The lack of centralized waste management and sanitation in karst environments increases the risk of aquifer contamination. Increase of impervious surfaces and loss of soil decreases recharge to the aquifer that contributes to tunnel flows. Figure 2. Tunnel Diquini and Source Mariani discharge with ENSO climate events, 1980 - 2018. II. Source Mariani Summary Source Mariani is currently the most distal source of water that supplies the CTE-RMPP water system. It is the largest naturally flowing spring and second largest single water source that supplies the Port-au-Prince municipal water system. When the pumping station is in operation, an average of ~19,000 m³/day is available to supply ~17% of total municipal production, and 24% of all spring flow supplying metropolitan Port-au-Prince region (based on CTE-RMPP data 2014-2018). The spring discharges from limestones that drain a portion of the Massif de La Selle carbonate aquifer system, west of the Riviere Froide and north of the Riviere Momance. The objective of this evaluation is to better understand the spring flow characteristics and the origin of the waters to guide future study of the Massif de la Selle aquifer system and to aid CTE- RMPP in water use planning, development, monitoring and protection. This investigation was accomplished using a combination of literature and data review, satellite and topographic imagery analysis, and field reconnaissance. A brief field mission to the spring was conducted in April 2019, including: (i) physical and chemical sampling, (ii) stable isotope sampling, (iii) chlorofluorocarbon (CFC) and sulfur hexafluoride (SF6) sampling, and (iv) visual observation of local geology. At a follow-up visit to the spring was conducted in January 2020 to verify more recent flow monitoring data received from CTE-RMPP, two nearby springs were subsequently document that represent ~25% of the overall flow from the Mariani spring system. 8 Water Budget 1. The long-term mean annual recharge rate in the karst terrain is estimated greater than 30% of annual precipitation. Recharge rates can be higher in years with tropical storms and hurricanes, and less than 15% in years with normal or low precipitation. 1. Aquifer storage of the ‘spring shed’ is estimated between 155 and 259 Mm³. 2. The limestone karst aquifer that feeds the spring is well mixed and has an average groundwater age of between 21 and 35 years based on a single sampling event. Spatial Distribution of Groundwater Recharge 1. The groundwater recharge area that contributes to the spring flow appears to be approximately 13.3 km² but may be larger. a. This uncertainty in the recharge area is largely due to the complexity of groundwater flow in karst environments and limited datasets regarding tracers and hydrochemistry. 2. The average recharge elevation is estimated at 580 m above mean sea level with a corresponding temperature of 22.7 C. This suggests the possibility that some spring flow may originate from distal zones in the regional carbonate aquifer such as within the Riviere Momance basin. Photo 3. The Source Mariani catchment. Photo 5. Limestone outcrop near Source Mariani. Photo 4. Source Mariani pumping station. 9 Figure 3. Geologic Map of Interpreted Catchment Area of Source Mariani Flow Characteristics 1. Recharge to Massif de la Selle aquifer system appears to be largely affected by high intensity and high-volume rainfall events such as hurricanes and tropical storms. There appears to be a 3-to-7-year cycle of recharge trends partially influenced by El Niño and La Niña events. 2. Spring discharge displays mild seasonal variability, with monthly average flows typically ranging between 14,500 and 25,000 m³/d with an average of 19,500 m³/d. a. Instantaneous (daily) flows display greater variability, ranging from 7,600 to 30,700 m³/d. b. Based on the spring catchment infrastructure as observed in 2019, spring flow is measured from a single water meter. Since this method does not account for overflow, some high spring flows could be underreported. 3. The spring flow is most vulnerable to extended periods of average or below average precipitation and to consecutive years without high intensity rainfall periods such as tropical storms and hurricanes. 10 Figure 4. (i) Source Mariani and Tunnel Diquini flow compared with annual precipitation, (ii) average monthly flow of Source Mariani by month. Groundwater / Surface Water Interaction 1. Source Mariani flows from the regional Massif de la Selle carbonate aquifer system. a. Source Mariani is the lowest elevation terrestrial outlet known for the aquifer and appears to emanate from a topographic exposure of the main aquifer lithology rather than as a contact spring. This may act to sustain flows even when higher elevation springs exhibit reduced flows. b. Source Mariani essentially serves as a drain for the western portion of the Massif de la Selle aquifer. 2. Source Mariani does not appear to have a significant hydraulic connection to the Riviere Momance or Riviere Froide. This is supported by the isotope and tracer sampling and analysis of recharge catchment size. a. This recharge characteristic and resulting flow regression that extends from 2014 to 2019, may foster perceptions that the spring flow has been decreasing over the long-term or that acute impacts have occurred. b. Historical flow data from 1925 and 1933 has similar flow rates as the present. 4. The cyclic and multi-annual recharge characteristics are important for water managers to understand in order to balance water use allocations from the different water sources of CTE-RMPP. 11 Aquifer Vulnerability 1. Due to the high permeability and rapid infiltration rates typical in karst limestone environments, the spring vulnerable to contamination. For example, fecal coliform, E. coli, and salmonella contamination was reported by Eptisa in March 2014. 2. Urbanization and land use changes in the hills south of the spring may result in negative impacts to water quality and flow. The lack of centralized waste management and sanitation in karst environments increases the risk of aquifer contamination. Increase of impervious surfaces and loss of soil decreases recharge to the aquifer that contributes to spring flow. III. Plaine du Cul-de-Sac Summary The Plaine du Cul-de-Sac (PCS) aquifer is one of the largest aquifers in Haiti and currently provides at least 25% of the water supply for Port-au-Prince from 26 municipal wells. The aquifer is also an important water supply for private, agricultural and industrial wells. A numerical groundwater flow model was developed to better understand the hydraulic parameters and behavior of the PCS aquifer and support water supply development planning for the Port-au-Prince metropolitan region. The model effort was preceded with data mining and research to support the model construction and calibration. The primary goal of the modeling exercise was to better understand i) the sustainable and renewable quantities of groundwater available from the aquifer, ii) the complex recharge dynamics, and iii) surface water/ groundwater interactions between lakes and river systems. MODFLOW¹ 2005 code was selected for modeling the PCS aquifer. ViewLog software from Earthfx Inc. was applied to build the model, this software directly integrates with the borehole database for building, developing and refining the model. Groundwater Vistas Advanced, version. 6 was used to run the model simulations and scenarios. 12 Calibrated Baseline Groundwater Flow Model Details 1. Aquifer extent of 363 km² 2. Maximum thickness greater than 200 meters. 3. Multi-layer aquifer system, silty sand, and sandy gravel layers. 4. Current pumping simulation: 71,600 m³/day from 141 wells, 26 of which are municipal wells. Groundwater Flow The groundwater flow shows similar trends as has been illustrated in previous reports. The hydraulic gradient is steepest in the southern limits of the aquifer where the Riviere Grise and Riviere Blanche enter the plain and recharge the aquifer. A groundwater divide bisects the aquifer in the east- central portion where groundwater flows either westward towards the ocean or north and eastward into Trou Caiman, Canal Boucambrou, and Lac Azuei. Figure 5. Simulated groundwater piezometry under steady-state conditions. ¹ MODFLOW is the United States Geological Survey (USGS) three-dimensional finite-difference groundwater model. 13 Figure 6. Conceptual hydrogeologic cross section of the Plaine du Cul-de-Sac aquifer. Takeaways and Insights Based on the steady-state model and initial model run the following observations are noted regarding the water balance: • Renewable recharge inputs to the aquifer are on the order of 135,000 m³/day. If current pumping is 71,600 m³/day, this would imply a 0.53, groundwater development ratio. • 83% of the aquifer inputs are from infiltration of the Riviere Grise and the Riviere Blanche, consistent with historical findings. • Canal Boucambrou appears to be a drain from the PCS aquifer. This relationship needs to be examined through investigation and monitoring. • Lac Azuei does not appear to receive a significant proportion of its water budget from the PCS aquifer, and in fact the simulation suggests 21 L/s (1,838 m³/day). Lac Azuei appears influenced by stream infiltration of the Riviere Blanche. Groundwater that discharges to the eastern portion of Canal Boucambrou may flow into Lac Azuei. The relationship between the aquifer, Lac Azeui and Canal Boucambrou be further investigated through studies and monitoring. 14 • Based on the steady state simulation and assumed pumping schemes, saltwater intrusion does not appear to be a major factor at present for the primary aquifer layer. Dry season stress periods may enhance the risk, and the shallow layer is most susceptible. The coastal area of the aquifer has few wells; further, there was limited data to calibrate the model along the coast.. • Trou Caiman appears to receive water from the PCS aquifer, at a range that the model simulation suggests of 45 L/s (3,890 m³/day). Model Scenario Results Summary Model scenarios suggest that impacts should be anticipated from both climate change and increased pumping. • Rehabilitation of existing wells and addition of new municipal wells may drawdown the water table, thus affecting nearby wells. This may also create a stronger gradient between the Riviere Grise and the aquifer. • Pessimistic climate change scenarios appear to have a regional impact on the aquifer, since this is largely sensitive due to a decrease in river flows that in turn reduce the volume of the river water available to infiltrate into the aquifer. • The modeling exercise indicates that the Riviere Grise and the Riviere Blanche are critical components of the aquifer and its ability to sustain groundwater abstraction and flows to surface water bodies. The recharge from the river drives the hydraulic gradient, replenishes the aquifer when there is pumping or climate change stress, and it mitigates saltwater intrusion risk in coastal areas. Aquifer impacts from schemes related to river diversions and/or dams need to be understood and mitigated. 15 IV. Conclusions Summary The three investigations were effective in advancing preliminary understandings of Tunnel Diquini, Source Mariani and the PCS aquifer. The investigations shared common challenges, as all were limited due to the scarcity of data and knowledge to perform detailed hydrogeological studies. Uncovered data was often poorly documented. Significant efforts were necessary to synthesize, verify and utilize the few datasets that were available to support these studies. The 40-year record of flow collected and maintained for Tunnel Diquini should be strongly commended. A key finding from these studies is the importance of the Massif de la Selle carbonate aquifer system. It is arguably Haiti’s most important aquifer system, as it is responsible for the provision of a significant proportion of water supply to Port-au-Prince due to its large springs and the added benefit of gravity. The rivers originating in the Massif supply the bulk of recharge to the Plaine du Cul-de-Sac, and perhaps the Plaine de Leogane aquifer as well. High intensity precipitation events and ENSO cycles appear critical for recharging both the bedrock and alluvial aquifers. RMPP resource quantities are especially vulnerable during El Nino periods and consecutive years without pulses of recharge from tropical storms and hurricanes. The water quality of the aquifer systems is also a concern due to changing land use and inadequate waste management and sanitation. Potential impacts on the aquifer from schemes related to river diversions and/ or dams need to be understood and mitigated given that there is a strong connection between the aquifers and rivers. Scientific characterizations of the aquifer systems in Haiti are poorly developed, largely due to the lack of monitoring and data availability. This study and future studies will continue to be limited without time-series/ temporal datasets on spring flows, river flows, groundwater abstraction, water quality, environmental isotopes and meteorological parameters. Establishing and strengthening hydrological and hydrogeological monitoring programs with systematic procedures for data management and dissemination is an important recommendation that spans not just the three study areas, but the country as a whole. Tunnel Diquini and Source Mariani These studies provide a preliminary basis to inform planning and decision-making regarding the use, sustainability, and protection of the sources, so they continue to be an important source of water supply in the future. Considering the regional importance of these water supplies, additional investments are warranted and fall into three categories: 1. Strengthening of flow, precipitation, and water quality monitoring programs are outlined for both sources and nearby rivers to address important data gaps and to strengthen the understanding of the springs and the associated aquifer system. For example, monitoring is required to better understand connectivity between Riviere Froide, the aquifer, and the Tunnel Diquini. 2. Water source protection and enhancement –recharge protection areas should be delineated and protected to preserve the quantity and quality of the waters. Public education, land use planning, zoning, and controlled development in these areas is necessary especially as Haiti’s population has grown and waste management and sanitation practices are lacking. 3. Further study of both sources is necessary. The delineation of recharge areas and understanding of interactions with river systems requires more detailed geological mapping and isotope/tracer studies. Monitoring is important to advance the understanding and characterization of the tunnel and the Massif de la Selle regional 16 aquifer that supports it. Using the data and findings in this study, the potential exists for source protection and enhancement programs in key zones of the tunnel watershed. Additional studies are necessary to better understand the interaction between the tunnel and the nearby Riviere Froide. If any hydraulic or significant watershed changes are proposed for the Riviere Froide, we recommend comprehensive studies to evaluate and quantify the tunnel’s impacts. Plaine du Cul-de-Sac The steady-state groundwater flow model presented serves as a good tool to support the next steps of groundwater development and management in a regional context. The model is structured to support steady- state simulations of various groundwater abstraction, environmental and climate change scenarios. The resulting model suggest a renewable groundwater resources’ use on the order of 130,000 m³/day, thus further validating the importance of the Riviere Grise and Riviere Blanche streamflow infiltrations the recharge and groundwater flow dynamics of the aquifer system. Although a significant volume of data was compiled to support model development and calibration, the quality and reliability of data is variable. Further, a limited quantity of time- series or temporal data was available for river/ stream stages and water levels in wells. The development of transient and stress period models instead of should be considered, but must be supported with additional data mining, and a focused monitoring campaign of surface water flows and groundwater levels. Scientific characterization needs to be strengthened in the northeast, east and southeast portions of the aquifer to better understand lithology and the surface and groundwater interactions related to Lac Azuei, Canal Boucambrou and Trou Caiman. These will support model refinement and result in a greater level of confidence for these zones of the aquifer. Saltwater intrusion risk in the coastal areas should also be further investigated with monitoring. The importance of temporal monitoring of water levels and water quality in wells is important to support groundwater flow modeling and simulations. Temporal monitoring of flow and stage along multiple reaches of the Riviere Grise, Riviere Blanche and Canal Boucambrou is also important considering the three systems’ relevance in the aquifer’s dynamics. Well pumping estimates and monitoring also present a significant data gap that could be addressed through future activities, in the same manner that the estimation of current aquifer-wide abstraction was based on limited data. 17 HYDROGEOLOGICAL INVESTIGATION OF TUNNEL DIQUINI Characterization of Hydrology and Guidance for Source Monitoring and Protection Department Ouest, Republic of Haiti Prepared for: Inter-American Development Bank & DINEPA Prepared by: Northwater International and Rezodlo S.A. Final Report October 2018 Note: Additional data collection and research since this study was completed warrants revisiting and updating some of the analysis and findings of this report. 18 Keywords Tunnel Diquini, Plaine du Cul-de-Sac; Groundwater; Haiti; Port au Prince; hydrogeology; water supply; Massif de la Selle Latitude, Longitude 18.517N, 72.393W Citation Northwater International and Rezodlo. 2018. Hydrogeological Characterization of Tunnel Diquini: Port-au-Prince, Haiti, Inter-American Development Bank, Technical Report, HA-T1239-P001 Original report in English, French translation available. Authors James K. Adamson, PG Javan Miner, EIT Pierre-Yves Rochat 19 Table of Contents EXECUTIVE SUMMARY 20 SECTION 1.0 - INTRODUCTION AND PHYSICAL SETTING 21 SECTION 1.1 - GEOLOGY 22 SECTION 2.0 - METHODS AND RESULTS 24 SECTION 2.1 - HYDROLOGY 25 SECTION 2.2 - WATER QUALITY AND HYDROCHEMISTRY 27 SECTION 2.3 - STABLE ISOTOPE AND TRACER 29 SECTION 2.4 - GROUNDWATER AGE 30 SECTION 2.5 - AQUIFER STORAGE 30 SECTION 2.6 - GROUNDWATER RECHARGE 30 SECTION 3.0 - DISCUSSION 32 SECTION 4.0 - RECOMMENDATIONS FOR CONTINUED ACTIVITIES 33 A - STRENGTHENING ONGOING MONITORING EFFORTS 33 B – WATER SOURCE PROTECTION, ENHANCEMENT AND RIVER MONITORING 34 C – FURTHER HYDROGEOLOGICAL CHARACTERIZATION 35 SECTION 5.0 - CONCLUSIONS 37 REFERENCES 38 20 EXECUTIVE SUMMARY Tunnel Diquini is the largest single source of water for the municipal water system of Port-au-Prince, with an average supply of 29,449 m³/day to its metropolitan water system, based on records from 2014-2018. The tunnel accounts for ~26% of all the municipal production of water, and for 37% of all the gravity-fed spring flow that supplies the metropolitan area. An investigation was performed to analyze the hydrology of the tunnel’s waters and better understand the origin of the flow and its relationship with the groundwater and surface water systems. Knowing this is important to guide future studies and monitoring the tunnel, and also to aid the water use planning and management by Centre Technique d’Exploitation de la Région Métropolitaine de Port-Au-Prince (CTE-RMPP). The research was undertaken using a combination of literature review, satellite and topographic imagery analysis, and field reconnaissance. A brief field mission inside the tunnel was conducted on April 15, 2018, including: (i) physical and chemical sampling, (ii) stable isotope sampling, (iii) chlorofluorocarbon (CFC) and sulfur hexafluoride (SF6) sampling, (iv) flow rate measurement, and (v) visual inspection of tunnel geology. Based on the study, the key results and conclusions are summarized below: Spatial Distribution of Groundwater Recharge 1. The groundwater recharge area that contributes to the tunnel flow appears to range between 22 and 55 km². a. This recharge area depends on the rate and duration of Riviere Froide leakage to the regional carbonate aquifer. b. The average recharge elevation is estimated at 650 m. above mean sea level, indicating the possibility that some of the tunnel flow may originate from river leakage from the Riviere Froide to the regional carbonate aquifer. Groundwater Budget 1. The long-term average annual recharge rate in the karst terrain is estimated at 26% of annual precipitation. During high intensity rainfall periods, the recharge rates are substantially above 26%, while during normal or low precipitation periods the recharge could be lower than 10%. 2. Aquifer storage relative to the tunnel is estimated between 265 and 327 million m³. 3. The limestone karst aquifer that feeds the tunnel is well mixed and has an average groundwater age of 26 to 32 years based on a single sampling event. Flow Characteristics 1. Recharge to the regional aquifer and particularly to the tunnel appears to be largely affected by high intensity and high-volume rainfall events such as hurricanes and tropical storms. There appears to be a 3-to-7-year cycle of recharge trends partially influenced by El Niño and La Niña events. 2. Tunnel discharge is seasonally variable, with recorded flows ranging from 11,085 to 73,265 m³/d, and a geometric mean for all known recorded flows of 27,987 m³/d. 3. The tunnel flow is most vulnerable to extended periods of normal precipitation and consecutive years without high intensity rainfall periods such as tropical storms and hurricanes. a. This recharge characteristic, combined with the resulting flow regression that can extend over periods of years, may foster perceptions that the tunnel flow has been decreasing over the long-term or that some events have had an acute impact on it. b. Limited historical data from 1959 suggests that dry season low-flow conditions 21 are comparable or perhaps lower than current low-flow conditions and strongly influenced by major recharge or drought events. c. The response time of the aquifer to major recharge events such as hurricanes may be shortening, possibly due to land cover and climatic changes. Connection to Regional Groundwater and Surface Water 1. Both the tunnel and Riviere Froide are connected to the same regional karst limestone aquifer, from where they both receive flow. The Riviere Froide may recharge the aquifer at various spatial and temporal extents, and this could result in a possible link between the tunnel and river system. a. Further study and monitoring is required to better understand the complex hydraulic links between the Riviere Froide, the regional aquifer and the tunnel. 2. Tunnel Diquini does not appear to have a hydraulic connection to the Riviere Momance. This is supported by the nature of geological structure and faulting. a. The EPG fault zone and a perpendicular fault appear to direct groundwater in the Momance basin either into the Riviere Momance or into the lower reaches of the Riviere Froide, below where recharge to the tunnel would likely occur. Aquifer Vulnerability 1. Due to the high permeability and rapid infiltration rates typical in karst limestone environments, the tunnel waters have high vulnerability to contamination. 2. Urbanization and land use changes in the hills south of the tunnel portal are considered the greatest risk to the tunnel water quality and flow. The lack of centralized waste management and sanitation, combined with the karst hydrogeology, significantly increases the risk of direct contamination of the aquifer and tunnel waters. Increase of impervious surfaces and loss of soil associated with urbanization increases runoff and decreases recharge to the aquifer that contributes to tunnel flows. a. Land use planning, zoning, and managed development of the area south of the tunnel portal is necessary in order to protect the tunnel water from future water quality and flow impacts. Conclusions and Recommendations This study provides a preliminary basis to inform planning and decision-making with regards to the sustainability and protection of Tunnel Diquini so that it continues to be an important water supply in the future. If further work is planned in the tunnel watershed or more information is needed concerning the tunnel, recommendations are provided at the end of the report about water source protection, compiling historical data, and monitoring of climate, flow and water quality. SECTION 1.0 - INTRODUCTION and Physical Setting This study is part of a coordinated effort to better understand the existing and potential water supplies to serve the metropolitan area of Port-au-Prince. The intent of this study is to determine the hydrology of the tunnel waters and better understand the origin of its flow and its relationship with groundwater and surface water systems. Specifically, it is important to better understand whether significant recharge of the tunnel occurs from either the Momance or Froide rivers. Tunnel Diquini was completed in 1940 by the J.G. White Engineering Corporation. It is 22 currently the largest single water source of the Port-au-Prince municipal water system. Tunnel flow accounts for approximately 24% of total municipal production, and 38% of all gravity-fed spring flow that supply the metropolitan region. Although design and construction documents for the tunnel were not available, it has been assumed that its primary target was an east-west trending normal fault, approximately 1.5 km south of the tunnel’s portal. It is believed that this fault drains a sizeable portion of the Massif de La Selle carbonate aquifer system in this area. The tunnel is reportedly 1.5-km in length along its main shaft. It appears to have an alignment approximately southward, although several minor changes in bearing were noted over the first several hundred meters from its entrance. At least one secondary tunnel branches from the main tunnel toward the southeast. The main tunnel is approximately 2.4 m wide with rectangular cross-section. Although minor roof collapse had occurred in several places, no constrictions to flow were observed. Below the normal fault, additional flow enters the tunnel from its sidewall, and roof seeps in fractures, merging with the main channel flowing toward the portal. The tunnel floor is rough, with fractured limestone bedding planes protruding into the water course. At the portal, the final length is concrete lined as the flow is channeled into a large pipe to supply the municipal system. The entire tunnel length is reportedly inspected annually by Mr. Mackenson Louis of CTE-RMPP. The tunnel watershed ranges from the portal elevation at 140 meters to over 1,800 meters in the upper reaches of the Riviere Froide watershed. Average annual rainfall ranges from 1,400 mm/year near the portal to 2,100 mm/year in the upper reaches of the watershed. Land cover in the watershed is variable, with steeper slopes tending to be covered with scrub and flatter areas used for subsistence agriculture. Woodring (1924) described the watershed above Source Diquini as primarily scrub vegetation, indicating the possibility that land cover has not changed considerably in this watershed over the last 100 years. Given this land use history, it is possible that the hydrology of the area of study had largely adjusted to deforestation by the early years of the tunnel. Section 1.1 - Geology The geology of the tunnel area is primarily composed of carbonates that range from lower-Miocene to Paleocene age. Most of the tunnel appears to be bored through upper to middle Eocene-age limestones that are hard and well bedded with low bedding attitude. In the area of the tunnel portal, beds of limestone were observed to be near horizontal. To the south of the portal, the Eocene limestones are dissected by fault-controlled valleys, and primarily consist of detrital limestones and chalky limestones of lower to upper Miocene age. The entire length of the tunnel is within the hanging wall fault block (stratigraphically offset by the normal fault that traverses east-west 1.5 km south of the portal). The northern wall of this fault is downthrown, and it likely impounds groundwater and fosters preferential groundwater flow paths along the fault trace through the higher permeability limestones. This is believed to be the primary target and main source of groundwater to the tunnel. Figure 1 displays the geology around the tunnel and associated watersheds based on country-wide geologic mapping (CERCG, 1989) and faults based on mapping by Pubellier (2000) and Cox et al (2011). Figure 2 displays a generalized geologic cross-section along the tunnel alignment southward to the Riviere Froide drainage at the Enriquillo- Plantain-Garden Fault. 23 Figure 1 - Geologic map of tunnel and associated watersheds with interpreted recharge zones. Figure 2 - Generalized cross section of tunnel geology and hydrology. 24 Section 2.0 - Methods and Results A brief field mission in the tunnel was conducted on April 15, 2018, including: (i) physical and chemical sampling, (ii) stable isotope sampling, (iii) chlorofluorocarbon (CFC) and sulfur hexafluoride (SF6) sampling, (iv) flow rate measurement, and (v) visual inspection of the tunnel’s portal geology. All activities at the tunnel were performed under the supervision of the caretaker of the tunnel, Mr. Mackenson Louis. The tunnel flow was measured at the tunnel’s portal just prior to the point where the flow leaves the open-rock channel with a Marsh-McBirney Flo-Mate 2000 portable electromagnetic velocity meter. The open- channel width was approximately 2.44 meters with an average water depth of 0.215 meters. The calculated flow rate was 1,300 m³/h, or 361 L/s, corresponding to a stage-height of 15.5 centimeters on the staff gauge affixed to the east side of the tunnel portal (this staff gauge does not extend to the channel bottom). The field team was escorted approximately 300 meters into the tunnel for geological inspection. The limestone appeared to be well-bedded, moderate to hard, and with a near-horizontal bedding attitude. Numerous seeps entered the tunnel from fractures and cavities daylighting the walls and ceiling along the 300 meters length inspected. Many of these seeps were less than 1 L/s, although several were estimated to flow at more than 5 L/s and one was estimated at 20 L/s. At approximately 150 meters from the tunnel portal, a secondary smaller tunnel enters the main tunnel from the east. This branch produces significantly cooler water than the main tunnel flow. This smaller tunnel was barricaded with cobble that surrounded a concrete pipe. Mr. Mackenson Louis reported that each November/December he walks the full length of the tunnel to inspect it. His father (now deceased) was the original caretaker of the tunnel since it was constructed. Based on their observations, they believe that the tunnel’s flow rate has been decreasing over the last several decades. Water was sampled several meters into the tunnel, as sampling farther into the tunnel was not feasible. Mr. Mackenson Louis believed that our sampling event was representative of a lower flow condition for the tunnel. A 12V sampling pump with flexible tygon tubing was used to collect low-flow samples where the tunnel flow was considered laminar. Samples for physical, chemical, and stable isotope analysis were collected by filling prepared sample bottles provided by the laboratories of analysis (First Environmental and Isotech). Chlorofluorocarbons (CFCs) and Sulfur Hexafluoride (SF6) were collected to age- date the groundwater discharging from the tunnel. Samples for CFC-11, CFC-12 and CFC-113 were collected using the glass bottle method with copper tubing as described by USGS and Reston Chlorofluorocarbon Laboratory. Samples for SF6 were collected using 1-Liter plastic-coated safety amber glass bottles with polyseal cone-lined caps, also applying methodologies developed by USGS and University of Utah Noble Gas Lab. All sampling bottles and excess air tubes for CFC’s and SF6 were provided by the Dissolved and Noble Gas Lab at the University of Utah. Samples for excess air analysis were also collected in ¼-inch copper tubes with clamps; these samples support correction of the SF6 data. Upon completion of sampling, all samples were wrapped in insulating materials and transported to the US for shipment to the analysis labs. Photo 1. View inside tunnel portal. 25 Photo 4. Example of near horizontal bedding. Photo 2. Secondary adit approximately 150 m from portal, noticeably colder flow. Photo 3. View of limestone geology looking toward tunnel portal. Photo 5. Example of seep in east tunnel wall. Section 2.1 - Hydrology Tunnel discharge varies significantly based on the intensity and duration of recharge events and the transit time through the aquifer. A discontinuous tunnel flow dataset was compiled from various sources spanning between 1980 and 2018. The data was primarily provided by Engineer Pierre Colon Geffrard of CTE-RMPP. Flow measurements were provided as average monthly flow data from 1980 to 2010, single monthly flow measurements estimated using a spinner velocity-meter from 2010 to 2014 and monthly flow measurements using an electromagnetic velocity-meter from 2014 to 2018. The project team was informed that a CTE-RMPP technician continues to measure flow rate at the tunnel each month. One older flow 26 Photo 6. Sampling for CFC’s and SF6. Photo 7. Existing staff gauge. measurement that was located for the tunnel, from August 1959 (Waite, 1960) recorded 217 L/s. Based on the available data, tunnel flow rates are seasonally and annually variable and range from 128 L/s to 848 L/s with a geometric mean of 324 L/s. Recharge to the regional aquifer and to the tunnel in particular appear to be largely affected by high intensity and high-volume rainfall events such as hurricanes and tropical storms, something that is typical of karst aquifers. There appears to be a 3-to-7-year cycle of recharge trends partially influenced by El Niño and La Niña events. Conversely, extended periods of declining flow rates occur during drought and El Niño years. Figure 3 shows fl