EFFECTS OF ARTIFICIAL RECHARGE ON GROUND-WATER QUALITY AND AQUIFER STORAGE RECOVERY

Ground water nitrate contamination and water level decline are common concern in Nebraska. Effects of artificial recharge on ground water qu ality and aquifer storage recovery (ASR) were studied with spreading b asins constructed in the highly agricultural region of the Central Pla tte, Nebraska. A total of 1.10 million m(3) of Platte River water rech arged the aquifer through 5000 m(2) of the recharge basins during 1992 , 1993, and 1994. This is equivalent to the quantity needed to complet ely displace the ground water beneath 34 ha of the local primary aquif er with 13 m thickness and 0.25 porosity. Successful NO3-N remediation was documented beneath and downgradient of the recharge basins, where NO3-N declined from 20 to 2 mg L-1. Ground water atrazine concentrati ons at the site decreased from 2 to 0.2 mg L-1 due to recharge. Both N O3-N and atrazine contamination dramatically improved from concentrati ons exceeding the maximum contaminant levels to those of drinking wate r quality. The water table at the site rose rapidly in response to rec harge during the early stage then leveled off as infiltration rates de clined. At the end of the 1992 recharge season, the water table 12 m d owngradient from the basins was elevated 1.36 m above the preproject l evel; however, at the end of the 1993 recharge season, any increase in the water table from artificial recharge was masked by extremely slow infiltration rates and heavy recharge from precipitation from the wet test growing season in over 100 years. The water table rose 1.37 m dur ing the 1994 recharge season. Resultant ground water quality and ASR i mprovement from the artificial recharge were measured at 1000 m downgr adient and 600 m upgradient from the recharge basins. Constant infiltr ation rates were not sustained in any of the three years, and rates al ways decreased with time presumably because of clogging. Scraping the basin floor increased infiltration rates. Using a pulsed recharge to c reate dry and wet cycles and maintaining low standing water heads in t he basins appeared to reduce microbial growth, and therefore enhanced infiltration.