ABSTRACT This study used dissolved organic carbon (DOC) carbon-14 (14C) to determine groundwater ages that were then used to calculate groundwater travel times in southern Nevada aquifers that have low organic content. These travel times are compared with the more standard dissolved inorganic (DIC) 14C method for determining groundwater ages and travel times. Groundwater travel times for aquifers in southern Nevada using DOC 14C are thousands of years shorter than DIC 14C travel times for groundwater flow outside of recharge areas along four of the five flow paths evaluated for this study. The DOC 14C travel times range from 2,300 to 2,900 years (yrs) as compared to DIC 14C travel times that range from 8,200 to 22,000 yrs (uncorrected ages) and 4,700 to 19,000 yrs (corrected ages). The DOC 14C groundwater travel times in carbonate-rock and volcanic-rock aquifers of southern Nevada are similar to travel times determined from: (1) hydrogeologic data; (2) observations of rapid high-level tritium transport at the Nevada National Security Site; and (3) 550,000-yr δ18O and δ13C global climate records for calcite precipitated in Devils Hole, Nevada at the end of one of the flow paths. DOC 14C travel time calculations need to account for fewer processes than DIC 14C for aquifers that contain little organic matter and do not have redox reactions. DOC 14C travel-times can be calculated directly from the DOC 14C data in these aquifers without corrections if dissolution of organic carbon and sorption and matrix diffusion of DOC 14C onto or into the aquifer matrix is minimal. Laboratory experiments showed that little organic carbon was being leached from aquifer rocks, sorption of organic carbon ranged from 4.3% sorbed in carbonate rocks to 0.5% sorbed in volcanic rocks, and matrix diffusion coefficients were slower in lower porosity carbonate rocks (1.7 x 10-7 cm2/s) than in higher porosity volcanic rocks (2.9 x 10-7 cm2/s). The lack of dissolution of organic carbon in study area aquifers is also supported by the decrease in DOC along flow paths and the DOC composition of groundwater changing little as groundwater flows from recharge areas into the adjacent valleys. In contrast, DIC 14C groundwater travel-time calculations must be corrected for complex chemical reactions and physical processes, including mineral/gas dissolution, mineral/gas precipitation/exsolution, cation exchange, and carbon isotopic exchange that can significantly change the amount of DIC 14C in groundwater along flow paths by processes other than radioactive decay. DIC 14C can also be affected by sorption and matrix diffusion, but these processes may, or may not, be captured in geochemical modeling of precipitation/dissolution and carbon isotopic exchange reactions. Corrected ages, therefore, represent maximum ages.

中文翻译：

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使用溶解有机碳的碳 14 确定低有机碳含水层中的地下水年龄和旅行时间

摘要 本研究使用溶解有机碳 (DOC) 碳 14 (14C) 来确定地下水年龄，然后将其用于计算有机含量低的内华达州南部含水层的地下水流动时间。将这些旅行时间与更标准的溶解无机 (DIC) 14C 方法进行比较，以确定地下水年龄和旅行时间。使用 DOC 14C 的内华达州南部含水层的地下水传播时间比 DIC 14C 沿本研究评估的五个流动路径中的四个流向补给区外的地下水流动的时间短数千年。DOC 14C 旅行时间范围为 2,300 至 2,900 年（年），而 DIC 14C 旅行时间范围为 8,200 至 22,000 年（未校正年龄）和 4,700 至 19,000 年（校正年龄）。内华达州南部碳酸盐岩和火山岩含水层中的 DOC 14C 地下水旅行时间与根据以下条件确定的旅行时间相似：(1) 水文地质数据；(2) 对内华达国家安全站点快速高水平氚传输的观测；(3) 550,000 年 δ18O 和 δ13C 全球气候记录，内华达州魔鬼洞在其中一条流动路径的末端沉淀出方解石。DOC 14C 走时计算需要考虑的过程比 DIC 14C 少，因为含水层几乎不含有机物且没有氧化还原反应。如果有机碳的溶解和 DOC 14C 在含水层基质上或向含水层基质中的吸附和基质扩散是最小的，则可以直接从这些含水层中的 DOC 14C 数据计算 DOC 14C 旅行时间，而无需进行修正。室内试验表明，从含水层岩石中浸出的有机碳很少，对有机碳的吸附范围从碳酸盐岩中吸附的 4.3% 到火山岩中吸附的 0.5% 不等，并且基质扩散系数在较低孔隙度的碳酸盐岩中较慢（1.7 x 10- 7 cm2/s) 高于孔隙率较高的火山岩 (2.9 x 10-7 cm2/s)。研究区含水层中有机碳缺乏溶解也得到了沿流动路径的 DOC 减少的支持，而且随着地下水从补给区流入相邻的山谷，地下水的 DOC 组成变化很小。相比之下，DIC 14C 地下水走时计算必须针对复杂的化学反应和物理过程进行校正，包括矿物/气体溶解、矿物/气体沉淀/溶出、阳离子交换、和碳同位素交换，可以通过放射性衰变以外的过程显着改变地下水中沿流动路径的 DIC 14C 的量。DIC 14C 也可能受吸附和基质扩散的影响，但这些过程可能会或可能不会在沉淀/溶解和碳同位素交换反应的地球化学模型中被捕获。因此，校正年龄代表最大年龄。