Groundwater 14C Age at the Point of Discharge in a Groundwater Dominated Stream: Sampling Below the Streambed in Nested Watersheds in the Nebraska Sand Hills (USA).

Abstract:

Tracer-based groundwater transit time distribution (TTD) studies have found significantly lower discharge of young groundwater (0-20 yr) than predicted by model-based TTDs. This research is a part of a multi-institution NSF project to scrutinize this discrepancy, comparing model-based and tracer-based TTDs in the High Plains Aquifer of the Nebraska Sand Hills. Work presented here focuses on identifying the ¹⁴C age of groundwater at the site. 86 groundwater samples were collected in/beneath the streambed at five locations (6, 13, 20, 40, and 99 km from a zero point near the headwaters) in nested watersheds of the Middle Loup River, utilizing a narrow, steel piezomanometer with a five cm screen. Nested watershed areas range from about 14 – 3900 km². Using chemical and isotopic data from our samples and from the literature, we applied both “single-sample” geochemical models and NETPATH to estimate the ¹⁴C ages of groundwater discharging into the stream. Of the 86 samples, ¹⁴C-DIC ranged from 28.3 to 117.6 percent modern carbon (pmc); 18 samples had ¹⁴C > 100 pmc, likely indicating the presence of “bomb carbon.” An additional 13 samples contained ³H in excess of 0.4 TU, likely indicating they had recharged since about 1950. Thus, 31 samples in all (at 6, 13, and 20 km) were likely too young for ¹⁴C age dating. δ¹³C ranged from -15.15 to -5.87‰. 34 of 36 samples with ¹⁴C < 70 pmc had δ¹³C clustered near -8‰. Groundwater at 6, 13, and 20 km had average ¹⁴C = 92 pmc, while ¹⁴C at 40 and 99 km was much lower, averaging 54 pmc. The trend with distance downstream was not simple and monotonic: groundwater at 20 km surprisingly had both high average ¹⁴C and the highest mean calcite saturation index (97.1 pmc and -0.51, respectively). Groundwater ages at 6, 13, and 20 km were almost all in the range of 5 to 50 yr based on ³H/³He, far younger than at the two downstream locations. Initial application of the Han and Plummer model indicates that groundwater age is up to about five thousand years at 40 and 99 km. These model runs were based on ¹⁴C = 100 pmc and δ¹³C = -13.0‰ for soil CO₂, and ¹⁴C = 0 pmc and δ¹³C = -2‰ for calcite. The ¹⁴C results show the importance of assessing the presence of old groundwater in TTD studies; absent these results, we would have a very different impression of the groundwater TTD in the watershed of the Middle Loup River.