CONSTRUCTED WETLANDS AS A COMPONENT OF OPTIMAL GROUND-WATER MANAGEMENT

The Mississippi River Valley alluvial aquifer (alluvial aquifer) has experienced heavy ground-water usage, resulting in falling water-table levels over large areas and decreased water yields to existing wells. Ground-water flow models were developed for the north and south portions of the alluvial aquifer, primarily in eastern Arkansas. Conjunctive-use optimization modeling was done to assist water managers by estimating the maximum amount of ground water that hypothetically could be withdrawn from wells and streams without violating hydraulic-head or stream-discharge constraints. Modeling results indicate that pumping at 1997 withdrawal rates is unsustainable if water levels within unconfined aquifers must everywhere be at or above half the thickness of the formation. One option to better manage the water resources of the alluvial aquifer is to divert surface water to constructed wetlands in areas with depressed water levels, then storing some of this water in the ground-water system through artificial recharge. Passive artificial recharge involves the infiltration of surface water into the ground-water system by gravity flow through a series of wells, galleries, or permeable beds. However, earlier attempts at performing artificial recharge in specially constructed wells resulted in disappointing flow rates. The conclusions of these tests were that the performance of recharge through wells was adversely impacted by (1) air entrapment within the aquifer, (2) sediments lodging within the aquifer, (3) bio-fouling of the well screen and adjacent sediment, and (4) swelling of clays because of incompatible water chemistry. Constructed wetlands may provide a cost-effective, passive means of pre-cleaning surface water derived from a combination of rivers, canals, and tail-water recovery systems, prior to artificially recharging this water into the ground-water system.