GROUND-WATER AGE AND NITRATE DISTRIBUTION WITHIN A GLACIAL AQUIFER BENEATH A THICK UNSATURATED ZONE

The impact on ground water quality from increasing fertilizer applicat ion rates over the past 40 years is evaluated within a glacial aquifer system beneath a thick unsaturated zone. Ground water ages within the aquifer could not be accurately determined from the measured distribu tion of H-3 and as a result, chlorofluorocarbon (CFC) and H-3/He-3 dat ing techniques were applied. Beneath a 25 m thick unsaturated zone, gr ound water ages based on CFC-11 concentrations were greater than H-3/H e-3 ground water ages by 6 to 10 years, due to the time lag associated with the diffusion of CFCs through the unsaturated zone, Using the co rrected CFC-11 and H-3/He-3 ground water ages and the estimated travel time of 3H within the unsaturated zone, the approximate position of g round water recharged since the mid-1960s was determined, Nitrate conc entrations within post mid-1960s recharge were generally elevated and near or above the drinking water limit of 10 mg-N/L. In comparison, pr e mid-1960s recharge had nitrate concentrations <2.5 mg-N/L. The eleva ted NO3- concentrations in post mid-1960s recharge are attributed main ly to increasing fertilizer application rates between 1970 and the mid -to late 1980s. Anaerobic conditions suitable for denitrification are present within pre mid-1960s recharge indicating that removal of DO is a slow process taking tens of years, Over the next 10 to 20 years, ni trate concentrations at municipal well fields that are currently captu ring aerobic ground water recharged near the mid-1960s are expected to increase because of the higher fertilizer application rates beginning in the 1970s and 1980s.