USE OF RADIUM ISOTOPES AND WATER CHEMISTRY TO DETERMINE PATTERNS OF GROUNDWATER RECHARGE TO OTSEGO LAKE, OTSEGO COUNTY, NY

The chemistry and isotopic composition were measured seasonally in Otsego Lake, NY, and its three major tributaries during 2004. The lake was sampled at 14 locations at a depth of 16.5m. Radium isotopes were trapped at each site by pumping 200-400L of water through tandem micropore and MnO₂-impregnated filters. Other constituents were measured directly from water samples. About 75% of the sampling and lab analyses have been completed.

The main streams that feed the lake are located mainly on carbonate strata (Devonian Helderberg and Onondaga Groups). Stream and lake samples are supersaturated with calcite and slightly undersaturated with dolomite. Mean SI values in the streams are: SIc=+0.45 and SId=-0.23, where SI=log [IAP/K]. In the lake, the mean SIc=+0.33 and SId=-0.25. Mean P_CO2=0.0013 atm in both the lake and its tributaries. The positive SIc is apparently caused by loss of CO₂ from inflowing groundwater. Carbonate rocks are exposed only at the northern end of the lake, but calcite supersaturation in the lake shows the significant control of carbonates on its chemistry.

Radium isotopes are derived mainly from U and Th in rock and sediment. ²²⁸Ra and ²²⁶Ra have long half-lives (5.8 and 1601 years). ²²⁴Ra and ²²³Ra have short half-lives (3.6 and 11.1 days). Ratios of short to long half-life isotopes decrease with residence time in surface water (=time since water was released from the ground). High ²²⁸Ra/²²⁶Ra (approaching 1.0) indicates lengthy contact with detrital materials. Values less than roughly 0.5 are typical for local carbonates. The mean ²²⁸Ra/²²⁶Ra is 0.69 in tributary streams and 0.75 in the lake. Despite the high carbonate levels, these ratios show much groundwater contact with detrital materials. Tributaries have high ²²³Ra/²²⁶Ra and ²²⁴Ra/²²⁸Ra, indicating abundant groundwater recharge. Low Ra activities, as well as similarity of δ¹⁸O and δD between streams and contemporaneous rainfall, suggest rapid groundwater flow typical of carbonates. Two sites in the lake have a high ²²³Ra/²²⁶Ra of 0.014 (twice the mean of the others), which suggests local groundwater inputs. One at the northern end is in a favorable place for karst springs. The other is near the southern end of the lake, where upward recharge may occur through glacial gravels.