Joseph A. Magner | Geology Reports

USGS2013

Air - water temperature relationships in the trout streams of southeastern Minnesota’s carbonate - sandstone landscape

Carbonate-sandstone geology in southeastern Minnesota creates a heterogeneous landscape of springs, seeps, and sinkholes that supply groundwater into streams. Air temperatures are effective predictors of water temperature in surface-water dominated streams. However, no published work investigates the relationship between air and water temperatures in groundwater-fed streams (GWFS) across watersheds. We used simple linear regressions to examine weekly air-water temperature relationships for 40 GWFS in southeastern Minnesota. A 40-stream, composite linear regression model has a slope of 0.38, an intercept of 6.63, and R 2 of 0.83. The regression models for GWFS have lower slopes and higher intercepts in comparison to surface-water dominated streams. Regression models for streams with high R 2 values offer promise for use as predictive tools for future climate conditions. Climate change is expected to alter the thermal regime of groundwater-fed systems, but will do so at a slower rate than surface-water dominated systems. A regression model of intercept vs . slope can be used to identify streams for which water temperatures are more meteorologically than groundwater controlled, and thus more vulnerable to climate change. Such relationships can be used to guide restoration vs . management strategies to protect trout streams.

Minnesota↗

USGS1996

Nitrate in groundwater and water sources used by riparian trees in an agricultural watershed: A chemical and isotopic investigation in southern Minnesota

This study evaluates processes that affect nitrate concentrations in groundwater beneath riparian zones in an agricultural watershed. Nitrate pathways in the upper 2 m of groundwater were investigated beneath wooded and grass-shrub riparian zones next to cultivated fields. Because trees can be important components of the overall nitrate pathway in wooded riparian zones, water sources used by riparian trees and possible effects of trees on nitrate concentrations in groundwater were also investigated. Average nitrate concentrations in shallow groundwater beneath the cultivated fields were 5.5 mg/L upgradient of the wooded riparian zone and 3.5 mg/L upgradient of the grass-shrub zone. Shallow groundwater beneath the fields passed through the riparian zones and discharged into streams that had average nitrate concentrations of 8.5 mg/L (as N). Lateral variations of δD values in groundwater showed that mixing among different water sources occurred beneath the riparian zones. In the wooded riparian zone, nitrate concentrations in shallow groundwater were diluted by upwelling, nitrate-poor, deep groundwater. Upwelling deep groundwater contained ammonium with a δ 15 N of 5‰ that upon nitrification and mixing with nitrate in shallow groundwater caused nitrate δ 15 N values in shallow groundwater to decrease by as much as 19.5‰. Stream water penetrated laterally beneath the wooded riparian zone as far as 19 m from the stream's edge and beneath the grass-shrub zone as far as 27 m from the stream's edge. Nitrate concentrations in shallow groundwater immediately upgradient of where it mixed with stream water averaged 0.4 mg/L in the wooded riparian zone and 0.8 mg/L near the grass-shrub riparian zone. Nitrate concentrations increased toward the streams because of mixing with nitrate-rich stream water. Because nitrate concentrations were larger in stream water than shallow groundwater, concentrated nitrate in the streams cannot have come from shallow groundwater at these sites. Water sources of riparian trees were identified by comparing δD values of sap water, soil water, groundwater, and stream water. Soil water was the main water source for trees in the outer 4 to 6 m of one part of the wooded riparian zone and outer 10 m of another part. Groundwater was a significant water source for trees closer to the streams where the water table was less than about 2.1 to 2.7 m below the surface. No evidence was found in the nitrate concentration profiles that trees close to the streams that took up groundwater through their roots also took up nitrate from groundwater. The lack of such evidence is attributed to the nitrate concentration profiles being insufficiently sensitive indicators of nitrate removal by trees.

Minnesota↗