Dissolved Organic Carbon 14C in Southern Nevada Groundwater and Implications for Groundwater Travel Times

doi:10.2172/1287225

GSTDTAP > 地球科学

DOI	10.2172/1287225
报告编号	45268
来源ID	OSTI ID: 1287225
	Dissolved Organic Carbon 14C in Southern Nevada Groundwater and Implications for Groundwater Travel Times
	Hershey, Ronald L.; Fereday, Wyall; Thomas, James M
	2016-08-01
出版年	2016
页数	59
语种	英语
国家	美国
领域	地球科学
英文摘要	Dissolved inorganic carbon (DIC) carbon-14 (¹⁴C) ages must be corrected for complex chemical and physical reactions and processes that change the amount of ¹⁴C in groundwater as it flows from recharge to downgradient areas. Because of these reactions, DIC ¹⁴C can produce unrealistically old ages and long groundwater travel times that may, or may not, agree with travel times estimated by other methods. Dissolved organic carbon (DOC) ¹⁴C ages are often younger than DIC ¹⁴C ages because there are few chemical reactions or physical processes that change the amount of DOC ¹⁴C in groundwater. However, there are several issues that create uncertainty in DOC ¹⁴C groundwater ages including limited knowledge of the initial (A₀) DOC ¹⁴C in groundwater recharge and potential changes in DOC composition as water moves through an aquifer. This study examines these issues by quantifying A₀ DOC ¹⁴C in recharge areas of southern Nevada groundwater flow systems and by evaluating changes in DOC composition as water flows from recharge areas to downgradient areas. The effect of these processes on DOC ¹⁴C groundwater ages is evaluated and DOC and DIC ¹⁴C ages are then compared along several southern Nevada groundwater flow paths. Twenty-seven groundwater samples were collected from springs and wells in southern Nevada in upgradient, midgradient, and downgradient locations. DOC ¹⁴C for upgradient samples ranged from 96 to 120 percent modern carbon (pmc) with an average of 106 pmc, verifying modern DOC ¹⁴C ages in recharge areas, which decreases uncertainty in DOC ¹⁴C A₀ values, groundwater ages, and travel times. The HPLC spectra of groundwater along a flow path in the Spring Mountains show the same general pattern indicating that the DOC compound composition does not change along this flow path. Although DOC concentration decreases from recharge-area to downgradient groundwater, the organic compounds are similar, indicating that DOC ¹⁴C is unaffected by other processes such as microbial degradation. A small amount of organic carbon was leached from crushed volcanic and carbonate aquifer outcrop rock in rock-leaching experiments. The leached DOC was high in ¹⁴C (75 pmc carbonate rocks, 91 pmc volcanic) suggesting that the leached DOC likely came from microbes in the rock samples. The small amount of DOC and high ¹⁴C indicates that the amount of old organic carbon in these rocks is low so there should be minimal impact on groundwater DOC ¹⁴C ages. Based on the results from this study, DOC ¹⁴C ages do not require additional corrections. Several correction models were applied to DIC ¹⁴C ages to correct for water-rock reactions along two carbonate and two volcanic flow paths and the corresponding travel times were compare to DOC ¹⁴C travel times. The DOC ¹⁴C travel times were hundreds to thousands of years shorter than uncorrected and corrected DIC ¹⁴C travel times except for the upper section of one carbonate flow path. DOC ¹⁴C travel times ranged from 400 to 5,400 years as compared to DIC ¹⁴C that ranged from modern to 20,900 years. The DIC ¹⁴C ages are greatly influenced by carbonate mineral and gas reactions and other processes such as matrix diffusion, isotope exchange, or adsorption, which are not always adequately accounted for in DIC ¹⁴C groundwater age correction models.
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来源平台	US Department of Energy (DOE)
引用统计
文献类型	科技报告
条目标识符	http://119.78.100.173/C666/handle/2XK7JSWQ/7201
专题	地球科学
推荐引用方式 GB/T 7714	Hershey, Ronald L.,Fereday, Wyall,Thomas, James M. Dissolved Organic Carbon 14C in Southern Nevada Groundwater and Implications for Groundwater Travel Times,2016.