Water Quality and Agriculture


 By Maureen Hackett
 Irrigation Research Assistant
 C.V.I. Agricultural Experiment Station

 Water is oT great importance to the survival of every living
thing on earth. Man can live without food for 2-3 weeks, but
without water for only 34 days. The growth of plants is also
dependent on a source of water. For both man and plants, the
quality of water is as important as the quantity.

 The quality is determined by the natural features of the
area (geology, climate) and also man's influence on the water
cycle. In St. Croix there are four potential sources of water:
 1. Groundwater (wells)
 2. Potable water desalinizedd)
 3. Reclaimed Wastewater
 4. Impounded Surface Runoff (dams)

 According to Jordan in the 1975 V.I. Geological Survey,
in an average year about 40 inches of rain falls on St. Croix.
Approximately 90% of this returns to the atmosphere by evapo-
transpiration, streamflow to the sea accounts for about 1 inch,
and another 1-3 inches infiltrates the soil to eventually become
part of the groundwater system.


 600 SOUTH SOUTHEAST -

 500


 400


 300
 CARIBBEAN SEA
 /SOUTH SHORE OF ST CROIX
 200 GOLD
 z / AWWTP FAIR PLAINS

 S100 WELL FIELD
 >

 IJ 0
 ALLUVIUM .

 100 :::::: KINGSHILL MARL :


 200 (BLUE C


 300


GROUNDWATER
 The geology and soil structure are vital factors in deter-
mining the quality of groundwater in an island. Rain falling on
the ground contains gases and solids in solution. Water passing
through the unsaturated soil moisture zone picks up carbon diox-
ide (C02) from the soil air and thus becomes more acid. This acid
water, coming in contact with soil particles or bedrock, takes
certain mineral salts into solution. If the soil is well drained, the
eventual groundwater may contain a substantial amount of dis-
solved materials.

 Due to the varied geological formations on the island, the
quality of groundwater is also variable. The two major types of
rock found on St. Croix are sedimentary and igneous. Sedimentary
rock has a greater effect on groundwater since it is more easily
dissolved. The formations of marl and limestone in the central
plain were previously below sea level. Wells in the marl hills or
alluvial plains tend to produce a large quantity of water, but it is
usually heavily mineralized. Generally, wells located in the igneous
hills have low production, but produce water of a good quality
(low in dissolved minerals).

 Evapotranspiration is the most influential factor in the
quality of groundwater in St. Croix. Rain showers are often ':


I--j---- NORTH NORTHWEST


0 4,000 8,000 12,000 16,000 20,000
 HORIZONTAL DISTANCE (ft)
 Geological cross section of the coastal plain on St. Croix.
 (Source: Wastewater Reclamation Project by Oscar Krisen Buros, June 1976 EPA 600/2 /6 -134).