Using tracer-derived groundwater transit times to assess storage within a high-elevation watershed of the upper Colorado River Basin, USA

Georgek, Jennifer L.; Kip Solomon, D.; Heilweil, V. M.; Miller, Matthew P.

Using tracer-derived groundwater transit times to assess storage within a high-elevation watershed of the upper Colorado River Basin, USA | Utilisation des temps de transit des eaux souterraines, obtenus par traçage, pour évaluer le stockage du bassin amont du Colorado, USA Uso de tiempos de tránsito del agua subterránea derivados de trazadores para evaluar el almacenamiento en una cuenca hidrográfica de alta elevación en la Cuenca Alta del Río Colorado, EE.UU 采用由示踪剂得到的地下水经过时间评价美国上科罗拉多河盆地一个高海拔流域内的储水量 Usando tempo de trânsito das águas subterrâneas derivado de traçadores para avaliar o armazenamento dentro de uma bacia de alta altitude na Bacia do Alto Colorado, EUA

2018

Georgek, Jennifer L. | Kip Solomon, D. | Heilweil, V. M. | Miller, Matthew P.

Previous watershed assessments have relied on annual baseflow to evaluate the groundwater contribution to streams. To quantify the volume of groundwater in storage, additional information such as groundwater mean transit time (MTT) is needed. This study determined the groundwater MTT in the West Fork Duchesne watershed in Utah (USA) with lumped-parameter modeling of environmental tracers (SF₆, CFCs, and ³H/³He) from 21 springs. Approximately 30% of the springs exhibited an exponential transit time distribution (TTD); the remaining ~70% were best characterized by a piston-flow TTD. The flow-weighted groundwater MTT for the West Fork watershed is about 40 years with approximately 20 years in the unsaturated zone. A cumulative distribution of these ages revealed that most of the groundwater is between 30 and 50 years old, suggesting that declining recharge associated with 5–10-year droughts is less likely to have a profound effect on this watershed compared with systems with shorter MTTs. The estimated annual baseflow of West Fork stream flow based on chemical hydrograph separation is ~1.7 × 10⁷ m³/year, a proxy for groundwater discharge. Using both MTT and groundwater discharge, the volume of mobile groundwater stored in the watershed was calculated to be ~6.5 × 10⁸ m³, or ~20 m thickness of active groundwater storage and recharge of ~0.09 m/year (assuming porosity = 15%). Future watershed-scale assessments should evaluate groundwater MTT, in addition to annual baseflow, to quantify groundwater storage and more accurately assess watershed susceptibility to drought, groundwater extraction, and land-use change.

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