Relative risks of groundwater-quality degradation near selected California oil fields are estimated by examining spatial and temporal patterns in chemical and isotopic data in the context of groundwater-age categories defined by tritium and carbon-14. In the Coastal basins, western San Joaquin Valley (SJV), and eastern SJV; 82, 76, and 0% of samples are premodern (pre-1953 recharge), respectively; and 3, 0, and 31% are modern (recharged during or after 1953), respectively. Carbon-14 and helium-4 data indicate most premodern samples are 1000 to 10,000 (33%) or >10,000 (50%) years old. Organic chemicals that could be associated with deeper hydrocarbon reservoirs (e.g. thermogenic gases and benzene) occur most frequently in premodern groundwater, suggesting premodern groundwater has a higher risk of degradation from upward migration of hydrocarbons than modern and mixed-age groundwater. Low sulfate concentrations in some premodern groundwater containing high thermogenic-methane concentrations (>28 mg/L) indicate methane attenuation associated with sulfate reduction can be limited in premodern groundwater. The more common occurrence of manufactured compounds, like tetrachloroethene, in modern and mixed-age groundwater than in premodern groundwater indicates modern and mixed-age groundwater has a higher risk of degradation from land-surface sources than premodern groundwater. Time-series data for chloride in groundwater affected by disposal of oil-field water in unlined ponds indicate some modern and mixed-age groundwater are susceptible to chemical migration within 2–3 km of surface sources. Timescales for diluting chloride concentrations in groundwater with fresh recharge once disposal ponds are decommissioned are shorter in mixed-age groundwater with large fractions of modern water (9–14 years in one example) than in mixed-age groundwater with large fractions of premodern water (no evidence of dilution after 12 years of monitoring in one example). The presence of predominantly premodern groundwater in the Coastal basins and western SJV indicates these areas have relatively high risk from upward migration of hydrocarbons, reduced methane attenuation capacity, and long dilution times, whereas predominantly modern- and mixed-age groundwater in the eastern SJV indicates this area has relatively high risk from chemical migration from land-surface sources and subsequent extensive spreading. Age-based characterizations of relative risk could inform the design of groundwater-monitoring programs near oil fields in terms of the spatial distribution of monitoring points relative to source areas and monitoring frequency and duration.

在由氚和碳 14 定义的地下水年龄类别的背景下，通过检查化学和同位素数据的空间和时间模式来估计选定加利福尼亚油田附近地下水质量退化的相对风险。在沿海盆地、圣华金河谷西部 (SJV) 和 SJV 东部；82%、76% 和 0% 的样本分别是前现代的（1953 年之前的充值）；和 3%、0% 和 31% 分别是现代的（在 1953 年期间或之后充电）。碳 14 和氦 4 数据表明，大多数前现代样本的年龄为 1000 至 10,000 (33%) 或 >10,000 (50%) 岁。可能与更深的碳氢化合物储层有关的有机化学品（例如热成因气体和苯）最常出现在前现代地下水中，表明前现代地下水比现代和混合时代地下水具有更高的碳氢化合物向上迁移的退化风险。一些含有高产热甲烷浓度 (>28 mg/L) 的前现代地下水中的低硫酸盐浓度表明与硫酸盐还原相关的甲烷衰减在前现代地下水中可能受到限制。在现代和混合时代地下水中比在前现代地下水中更常见的人造化合物，如四氯乙烯，表明现代和混合时代地下水比前现代地下水具有更高的地表来源退化风险。受无衬里池中油田水处理影响的地下水中氯化物的时间序列数据表明，一些现代和混合时代的地下水容易受到地表源 2-3 公里范围内的化学迁移。一旦处置池退役，用新鲜补给稀释地下水中氯化物浓度的时间尺度在现代水占很大比例的混合时代地下水中（在一个例子中为 9-14 年）比在前现代水比例很大的混合时代地下水中更短。在一个例子中，经过 12 年的监测后没有稀释的证据）。沿海盆地和 SJV 西部主要存在前现代地下水表明这些地区存在碳氢化合物向上迁移、甲烷衰减能力降低和稀释时间长的风险相对较高，而 SJV 东部主要是现代和混合时代的地下水表明该地区具有相对较高的来自地表源的化学迁移和随后的广泛扩散的风险。相对风险的基于年龄的特征可以为油田附近地下水监测计划的设计提供信息，包括监测点相对于源区的空间分布以及监测频率和持续时间。