Informing nitrate concentration trends: estimating groundwater residence time in a karstic, multiaquifer system using anthropogenic tracers (Minnesota, USA)

Interpretation of groundwater nitrate trends in the agricultural, karstic landscape of southeastern Minnesota, USA, has been confounded by an inadequate understanding of residence time. Temporal monitoring of alachlor ethane sulfonic acid (ESA) concentrations, a transformation product of the discontinued but commonly used agricultural herbicide alachlor from the 1970s and 1980s, was used to approximate post-1953 groundwater residence times. Statewide alachlor usage patterns were compared with corresponding ESA concentration trends measured from springs and wells. Nonparametric regressions and other methods were used to quantify residence times, which were compared with the results derived from atmospheric tracers. Corroboration between the multiple, independent age tracer methods suggests groundwater mixtures from shallow (<60 m) springs have apparent residence times of one to two decades. In contrast, springs in deeper settings have residence times ranging from two to four decades. Similar methods applied to domestic wells revealed analogous results across comparable hydrogeologic conditions. Residence times for springs and wells, across all age dating methods were significantly different by geologic setting and correlated with depth. Trend analysis combined with residence time estimates implies legacy sources of contaminants, such as nitrate, have not fully migrated through aquifer systems and that concentrations in certain springs, streams, and wells with older water will continue to increase until an equilibrium has been reached with current and historical land use. These results also provide insight into the challenges of measuring water quality response to improved land-use practices using monitoring sites with mixed groundwater residence times.