Summary of La Gonave, Haiti

Summary of La Gonave, Haiti - Geology and Hydrogeology
James K. Adamson, PG
August 22, 2013
1
Summary
The island of La Gonave is approximately 750 square kilometers in size and located o↵ the
coast of mainland Haiti. Approximately 115,000 people inhabit the island, about half of
the island population resides in the towns of Anse-a-Galets and Pointe-a-Racquette. The
remainder of the island is rural with small isolated communities.
Annual precipitation is estimated between 700 and 1,200 mm/year, with significantly
higher precipitation on the eastern portion of the island. The dry season on La Gonave
typically occurs from late October through April. La Gonave has little perennial surface
water with the exception of several springs. In the eastern portion of the island, many
springs are present due to stratigraphic and structural factors. The western portion of the
island has no inland springs, but many coastal karst springs that often produce brackish
water. A primary reason for the water scarcity on the western portion of the island is due
to the thick Pliocene and Pleistocene-aged coral limestones, which infiltrate a majority of
precipitation.
Figure 1 is a geologic map of La Gonave that was developed though the compilation
of the work of many geologic investigators over the past century that have contributed
to published or publicly available information. Some of these investigators include: J.
Adamson, R. Bien, D. Boisson, J. Butterlin, S. Dykstra, A. Momplaisir, H. Spruijt and J.
Troester.
The island is a complex northwest trending regional anticlinal feature, which exposes a
core of middle Eocene-aged limestone in the southeastern portion of the island. This unit is
the oldest rock found on La Gonave and blankets the highest portion of the island reaching
an elevation of approximately 780 meters. This limestone is believed to exist beneath the
entire island at variable depths; it is also the source rock for what is believed to be the
largest aquifer of the island. This aquifer is referred to as the Chien Content Aquifer,
named after the highest mountain on the island where the limestone is best exposed. This
aquifer is only a proven resource on the eastern portion of the island, its presence in western
La Gonave has not been verified scientifically.
Overlying the middle Eocene limestones are younger Eocene massive limestones that
are less prominent, referred to as the Matenoir complex. These are only observed at the
1
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15
Kilometers
Middle Eocene - La Pierre Limestone
Figure 1: Geologic Map of La Gonave
surface in the Nan Cafe and Matenoir areas. The lower Eocene-aged chalky limestone is
the next younger stratigraphic geologic sequence that covers a large portion of the island
and is prevalent in the Plaisance and Pointe-a-Racquette areas. The carbonate is chalky
with chert and is possibly over 1,200 meters thick in some areas of the island. Due to
the fine-grained and hydrologically impermeable nature of this geology, it makes finding
a viable groundwater supply very difficult in some areas. Overlying the Upper Eocene
chalky limestone are Miocene and Oligocene-aged limestones and marls, which are exposed
on the surface on the northeastern and eastern portions of the island. These units are
typically good source rock for groundwater and house the La Palmiste and La Palma
aquifers. During deposition of this geology, however, significant sea-cli↵ erosion of the
Eocene chalky limestones occurred which re-deposited fine grained sediments. As a result,
groundwater yields can be significantly variable in the Miocene and Oligocene strata.
The youngest and most prevalent rock on the island is Pliocene and Quaternary-aged
coral limestone. These limestones cover over half of the island and almost the entire western
portion. This limestone is believed to be up)"9 to 150 meters thick in some areas and exhibits
11
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karst topography. This limestone
however the
)"7 can)"6 be a)"8 good source rock for groundwater;
10
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karst nature of the rock promotes rapid seaward
drainage so 12groundwater is stored only
15
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43
44
where it is impounded, usually by hidden subsurface geologic structure and unconformities.
In many coastal areas, sea-level acts as hydraulic control on the groundwater located within
the coral limestone.
Western La Gonave is much di↵erent from a geology and hydrogeology perspective
than most of Haiti. The only area of Haiti that is similar is the far northwest (Mole St.
Nicholas, Bombardopolis, Mare Rouge, Baie de Henne). This geology is very complex
from groundwater exploration and drilling standpoint because groundwater resources are
restricted to preferential subsurface locations. The karst nature of the subsurface requires
advanced and experienced exploration and drilling programs.
2
Closing
There are several studies and work programs recently finished, or currently in progress
that will allow for a more comprehensive and thorough characterization of the island’s
groundwater resources. These include geophysical studies, drilling programs, hydrogeologic
analysis of existing wells and geologic field investigations. It is the hope of the author to
contribute towards compiling this new and historical information to ultimately develop a
comprehensive publication regarding the groundwater resources of La Gonave.
3