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Ground water age and nitrate distribution within a glacial aquifer beneath a thick unsaturated zone

Ground Water

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Abstract

The impact on ground water quality from increasing fertilizer application 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 distribution of 3H and as a result, chlorofluorocarbon (CFC) and 3H/3He dating techniques were applied. Beneath a 25 m thick unsaturated zone, ground water ages based on CFC-11 concentrations were greater than 3H/3He 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 corrected CFC-11 and 3H/3He ground water ages and the estimated travel time of 3H within the unsaturated zone, the approximate position of ground water recharged since the mid-1960s was determined. Nitrate concentrations within post mid-1960s recharge were generally elevated and near or above the drinking water limit of 10 mg-N/L. In comparison, pre mid-1960s recharge had nitrate concentrations <2.5 mg-N/L. The elevated NO3- concentrations in post mid-1960s recharge are attributed mainly 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, nitrate concentrations at municipal well fields that are currently capturing 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.The impact on ground water quality from increasing fertilizer application rates over the past 40 years is evaluated within a glacial aquifer system beneath a thick unsaturated zone. Beneath a 25 m thick unsaturated zone, ground water ages based on CFC-11 concentrations were greater than 3H/3He 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 corrected CFC-11 and 3H/3He ground water ages and the estimated travel time of 3H within the unsaturated zone, the approximate position of ground water recharged since the mid-1960s was determined. Nitrate concentrations within post mid-1960s recharge were generally elevated and near or above the drinking water limit of 10 mg-N/L. In comparison, pre mid-1960s recharge had nitrate concentrations <2.5 mg-N/L. The elevated NO3- concentrations in post mid-1960s recharge are attributed mainly 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.

Additional Publication Details

Publication type:
Article
Publication Subtype:
Journal Article
Title:
Ground water age and nitrate distribution within a glacial aquifer beneath a thick unsaturated zone
Series title:
Ground Water
Volume
36
Issue:
1
Year Published:
1998
Language:
English
Publisher:
Ground Water Publ Co
Publisher location:
Westerville, OH, United States
Larger Work Type:
Article
Larger Work Subtype:
Journal Article
Larger Work Title:
Ground Water
First page:
171
Last page:
180
Number of Pages:
10