VULNERABILITY OF GROUND WATER IN BLACK RIVER BASIN

art One: General Information to Define Vulnerability

able below shows basic information needed for ground water vulnerability assessment and mapping. This is the most comprehensive list; not all information can be easily collected nor entered into a Ground Water Information System (GWIS). The table serves as a reminder for data collection and data base design. Table is modified from the Guidebook on Mapping Groundwater Vulnerability. IAH, 1994.

TOPOGRAPHY

Elevation, slope variability of land surface; surface runoff paths, stream network density.

VEGETATIVE COVER

Land use, subsurface water pathways, recharge and discharge areas, fracture traces and lineaments, contaminant potential.

CLIMATOLOGY

Long records of precipitation, average temperature, humidity, solar radiation, evaporation, evapotranspiration; effective precipitation assessment.

SOILS

Thickness, structure, texture, mineralogy, chemical and physical properties, porosity, permeability, moisture, infiltration capacity.

HYDROLOGY

Streamflow discharge, hydrograph analysis, baseflow, flow ratio, water exchanges with underlying ground water systems.

HYDROGEOLOGY (Unsaturated zone)

Depth to water; thickness, lithostratigraphy, mineralogy, geometry, fracture index, karst index, effective porosity, and saturation ratio of surficial deposits; vertical effective permeability, effective flow velocity, infiltration rate index, net recharge.

HYDROGEOLOGY (Saturated zone)

Lithostratigraphy, geological structure, geometry, effective porosity, permeability type (primary or secondary), transmissivity, storativity, and hydraulic conductivity of an aquifer; aquifer type (unconfined, semiconfined, confined); water level fluctuations, hydraulic gradient, flow directions, effective flow velocity and discharge, ground water divides, exchanges with surface water bodies or/and adjacent aquifers.

WATER USE

Water-discharge points (spring, wells) and location of ground water extraction works; surface and ground water sources, distribution, and usage; yield and drawdown of pumping/dewatering plants, location and inflow rate of recharge systems.

HYDRO-CHEMISTRY

Physical and chemical properties of surface and ground water, chemical markers, isotope content, age and residence time of water, characteristic ratios; natural surface and ground water quality distribution.

CONTAMINANT FEATURES

Changes in water quality; contaminants present and their physical and chemical characteristics, concentration, half-life, persistence, mobility, dispersivity, cation exchange capacity, biodegradability, etc.

HUMAN IMPACT ON ENVIRONMENT

Extent of urban areas, location and type of industrial complexes, existing and potential contamination sources, potential contamination entries, main objects of protection.

art Two: General Vulnerability Criteria in Jamaica

Sea water intrusion: closeness to the coast, intrusion already advanced, too much ground water development.

Soil cover: thickness, permeability, filtration and sorption capacity.

Land use: agriculture with fertilizers (nitrates, phosphates, etc.).

Depth to ground water.

Ground water gradient, velocity of flow, residence time.

Alluvial deposits near rivers, rivers recharging aquifers, local cones of depression created near rivers.

Industrial developments, waste water ponds, industrial canals.

art Three: Rating System

Sea Water Intrusion

More than 5 km from the coast.

Within 2 and 5 km distance.

2 km coastal belt in pumped area.

Soil Cover

Zero cover thickness. Permeable materials (sand, gravel, limestone) exposed at the surface.

Clay, marl, mudstone, shale, and like, between 0 and 10 m thick, clean, impermeable; or more than 10 m thick but with sandy or loamy content.

Clay, marl, mudstone, shale, and like, more than 10 m thick; w/out sand, loam, or rock.

Land Use

Intensive agricultural developments, land fertilized with manure and industrial fertilizers.

Sporadic agricultural development, rain fed, use of industrial fertilizers only.

No agricultural developments.

Depth to Ground Water

Ground water normally less than 5 m below ground surface. This refers to unconfined aquifers.

Ground water between 15 and 5 m deep. Unconfined aquifers only.

Confined aquifers or ground water more than 15 m deep.

Ground Water Gradient, Velocity of Flow

Slow ground water flow, gradient less than 0.001 m/m; alluvial plain. Already established local cones of depressions due to heavy abstraction belong to this category.

Medium fast ground water flow; e.g. limestone at the slope break, within a plain.

Rapid natural ground water flow, short residence time. E.g. limestone, fractured, fissured, cavernous.

Alluvial Deposits near River

Aquifer receives water from the river during most of the year. A local abstraction has already created cones of depression below the river water surface.

River under flood stage may affect nearby aquifers.

River not affecting alluvium and shallow aquifers.

Industrial Developments, Waste Ponds

Aquifer development near existing waste ponds or established industries.

Industry remote, but an extended cone of depression may reach the places of industrial effluents, waste ponds, or local damping sites.

No industry within possible ground water development.

art Four: Specific Vulnerability Issues in Black River Basin

oastal belt.

The quality of ground water near the coast is generally good. There is only one water sample that displays the salinity of pure sea water. This is the well BR-010 at Black River Crane Road, with TDS 37,305 ppm and electrical conductivity of 45,370 microS/cm. The well is drilled near the coastline. The water sample was taken from the depth below the sea level. Even, the static water level appears to be slightly below the mean sea level.

The wells drilled near the coast at the Pedro Plains, that is the well BR-113 and the well BR-106 at Great Bay, display the water of acceptable quality. The well BR-106 has the electrical conductivity of 1430 microS/cm and the well BR-113 equal to 1130 microS/cm.

There may have been some wells drilled along the Black River within the town of Black River which may have been abandoned due to increased salinity on account of sea water intrusion.

ndustrial Waste Ponds

The contamination of ground water contained in limestone aquifer from industrial processing of bauxite into alumina has been discussed and documented in several places. The links are given below:

ontamination with Nitrates

Elevated content of nitrates has been noticed in certain areas. The US EPA standard for drinking water of 10 ppm (mg/l) has been surpassed in 13 out of 22 samples. The highest NO3 content of 142 ppm was noticed in the well BR-114 in Burnt Savannah.

High content of nitrates comes from agricultural practices. Limestone aquifer within the Black River basin is not protected with a thick soil cover. Very thin cover of permeable soil does not prevent irrigation water to dissolve fertilizers, pick up nitrates, and carry them downwards to the water table.