Testing the 14C ages and conservative behavior of dissolved 14C in a carbonate aquifer in Yucca Flat, Nevada (USA), using 36Cl from groundwater and packrat middensDétermination des âges au 14C et test du comportement conservative du 14C dissous dans un aquifère carbonaté du Yucca Flat, Nevada (Etats Unis d’Amérique) à partir du 36Cl des eaux souterraines et des amas fossiles de débris laissés par des rats.Pruebas de las edades 14C y comportamiento conservativo del 14C disuelto en un acuífero carbonático en Yucca Flat, Nevada (EEUU), usando 36Cl de agua subterránea y madrigueras de ratas利用 地下水和堆肥中的 36Cl测定(美国)内华达州Yucca Flat地区碳酸盐含水层中14C年龄和溶解14C的持恒行为Testando as idades de 14C e o comportamento conservativo do 14C dissolvido num aquífero carbonatado em Yucca Flat, Nevada (EUA), usando o 36Cl da água subterrânea e depósitos de acumulação de detritos criados por ratos

Corrected groundwater 14C ages from the carbonate aquifer in Yucca Flat at the former Nevada Test Site (now the Nevada National Security Site), USA, were evaluated by comparing temporal variations of groundwater 36Cl/Cl estimated with these 14C ages with published records of meteoric 36Cl/Cl variations preserved in packrat middens (piles of plant fragments, fecal matter and urine). Good agreement between these records indicates that the groundwater 14C ages are reasonable and that 14C is moving with chloride without sorbing to the carbonate rock matrix or fracture coatings, despite opposing evidence from laboratory experiments. The groundwater 14C ages are consistent with other hydrologic evidence that indicates significant basin infiltration ceased 8,000 to 10,000 years ago, and that recharge to the carbonate aquifer is from paleowater draining through overlying tuff confining units along major faults. This interpretation is supported by the relative age differences as well as hydraulic head differences between the alluvial and volcanic aquifers and the carbonate aquifer. The carbonate aquifer 14C ages suggest that groundwater velocities throughout much of Yucca Flat are about 2 m/yr, consistent with the long-held conceptual model that blocking ridges of low-permeability rock hydrologically isolate the carbonate aquifer in Yucca Flat from the outlying regional carbonate flow system.