Groundwater temperature and degassing in the Mad River subwatershed of Lake Huron

Abstract

Groundwater was measured 70 times in two years at 10 sites as it flowed 50m over an accumulation of travertine (CaCO3) before reaching the Mad River. At source, the groundwater was relatively cool (6.77±2.89°C), slightly acidic ( pH 6.86±0.22), and had a moderately high specific conductivity (606±51μS). Degassing was assessed from increases in pH, and CaCO3 deposition was assessed from decreasing conductivity. After flowing over the experimental site, degassing had increased pH to 8.04±0.16 (P<0.001), which was similar to river water (pH 8.07±0.30). Concurrently, CaCO3 deposition decreased conductivity to 577±43 μS (P<0.001) but this was still higher (P<0.001) than river water (494±72). Seasonal changes in air temperature affected the rate of degassing. The pH was correlated with air temperature (r=0.15, P<0.001) while conductivity was correlated with pH (r=−0.27, P<0.001), but no direct relationship of air temperature with conductivity was detected. Groundwater entering the river after atmospheric exposure had a fairly constant temperature (7.05±1.22°C) despite seasonal changes in air temperature, thus warming the river in winter (up to day 100 and after day 300) and cooling it in summer (from days 140 to 260). Degassing and CaCO3 deposition in springs without travertine was similar to that observed in the study stream over travertine. These groundwater inflows provide favorable pH and temperature conditions for brook trout.