STRONTIUM ISOTOPE SYSTEMATICS OF A LARGE IMPOUNDMENT LAKE

Strontium isotopes can be applied to identify important processes controlling surface water chemistry, such as sources of dissolved metals, Sr reservoirs, water-rock interaction and mixing dynamics. Lake Texoma is a large impoundment along the course of the Red River on the border between Texas and Oklahoma. The lake is fed by two river systems with typically different salinities, the Red River draining eastward, a generally more saline river with Total Dissolved Solid (TDS) as high as 3,000 mg/L (maximum salinities where the river flowing into the lake) and the Washita River entering Lake Texoma from the north with fresh waters (maximum TDS 900 mg/L). Lake waters are bottom released to the downstream Red River at Denison Dam. Water samples were collected at several locations throughout the lake in August 2003 and were analyzed for Sr concentrations and Sr isotopic ratios. The result of dissolved Sr (water samples were filtered with 0.45 µm membranes on site) shows descending Sr concentrations from 1.99 mg/L to 0.76 mg/L from the Red River arm to the Washita River arm. ⁸⁷Sr/⁸⁶Sr ratios range from 0.708694 ± 10 to 0.709233 ± 17, with a mean value of 0.708882. The highest ⁸⁷Sr/⁸⁶Sr ratio is found in water samples collected at the location close to the Washita River with the lowest Sr concentration, indicating more radiogenic water from Washita River inflow draining older igneous rocks (Cambrian). A vertical profile of ⁸⁷Sr/⁸⁶Sr was conducted with water samples collected in the main lake dam area, where the water column was 28 meters deep. ⁸⁷Sr/⁸⁶Sr ratios displayed differences between filtered and unfiltered samples at the same depth. Unfiltered samples showed homogeneous ⁸⁷Sr/⁸⁶Sr ratios around 0.7087 with depth, whereas filtered samples exhibit higher values of ⁸⁷Sr/⁸⁶Sr, with 0.708931 ± 22 at 5 meters, 0.709106 ± 18 at 24 meters and 0.708955 ± 17 at the bottom, respectively, though relatively low ⁸⁷Sr/⁸⁶Sr of 0.708694 ± 10 at 15 meters and 0.708861 ± 19 at 18 meters have been observed where the thermocline was developed. The differences of ⁸⁷Sr/⁸⁶Sr values between filtered and unfiltered samples suggest that Sr ions adsorbed to suspended solids and colloid surface are less radiogenic compared with dissolved species in the main lake water column of Lake Texoma, which may have important implications for Sr cycling in a binary mixing model.