STABLE ISOTOPE RESPONSE IN GROUND WATER TO SEASONAL ISOTOPE VARIATIONS IN PRECIPITATION

Stable hydrogen (δD) and oxygen (δ¹⁸O) isotope ratios were measured in five wells between January and July, 2014, sampled at high resolution. Water inputs during this time period included snowmelt as well as thunderstorms on the campus of Western Michigan University, the study area. Water table depths in these wells range from one to 25 meters. Two wells are overlain by urban soils that are generally well drained while three other wells have Entisols/urban complex soils. The measurements were made using a Triple Liquid Water Isotope Analyzer with a precision of 1 and 0.1‰ for δD and δ¹⁸O respectively. The δD and δ¹⁸O values range from -20 to -79 ‰ and -3 to -12‰ respectively.

Machavaram and Krishnamurthy (1994) established the mean δD and δ¹⁸O in precipitation in this study area. They found mean summer and winter isotopic values of -30.4 and -100 ‰ and -5.73 and -15.46 ‰ for δD and δ¹⁸O, respectively. In addition a local meteoric water line of the form δD = 7.5 δ¹⁸O + 13.6 was suggested. The two wells directly below urban soils responded within a few days to isotopic pulses from snowmelt and thunderstorms. These samples also fall close to the local meteoric water line. These are also the deepest wells. The remaining three wells are below the Entisols/urban complex soils. The shallowest well shows a less dramatic response to the isotopic pulse from snow melt. This might be due to the well screen being two meters below clay. These samples still fall close the local meteoric water line. The other two wells don’t show any obvious response to the isotopic pulse. They also indicate strong evaporation of the recharging waters based on the δD- δ¹⁸O slope of less than 4.

REFERENCES

Machavaram M. and Krishnamurthy R.V. (1994). Survey of Factors controlling the stable isotope ratios in precipitation in the Great Lakes region. Israel J of Earth Sciences, 43,195-202