U.S. Geological Survey, 1505 Ferguson Lane, Austin, TX 78754, United States.
U.S. Geological Survey, 5563 DeZavala, Ste. 290, San Antonio, TX 78249, United States.
Sci Total Environ. 2016 Oct 15;568:457-469. doi: 10.1016/j.scitotenv.2016.05.201. Epub 2016 Jun 14.
Many karst regions are undergoing rapid population growth and expansion of urban land accompanied by increases in wastewater generation and changing patterns of nitrate (NO3(-)) loading to surface and groundwater. We investigate variability and sources of NO3(-) in a regional karst aquifer system, the Edwards aquifer of central Texas. Samples from streams recharging the aquifer, groundwater wells, and springs were collected during 2008-12 from the Barton Springs and San Antonio segments of the Edwards aquifer and analyzed for nitrogen (N) species concentrations and NO3(-) stable isotopes (δ(15)N and δ(18)O). These data were augmented by historical data collected from 1937 to 2007. NO3(-) concentrations and discharge data indicate that short-term variability (days to months) in groundwater NO3(-) concentrations in the Barton Springs segment is controlled by occurrence of individual storms and multi-annual wet-dry cycles, whereas the lack of short-term variability in groundwater in the San Antonio segment indicates the dominance of transport along regional flow paths. In both segments, longer-term increases (years to decades) in NO3(-) concentrations cannot be attributed to hydrologic conditions; rather, isotopic ratios and land-use change indicate that septic systems and land application of treated wastewater might be the source of increased loading of NO3(-). These results highlight the vulnerability of karst aquifers to NO3(-) contamination from urban wastewater. An analysis of N-species loading in recharge and discharge for the Barton Springs segment during 2008-10 indicates an overall mass balance in total N, but recharge contains higher concentrations of organic N and lower concentrations of NO3(-) than does discharge, consistent with nitrification of organic N within the aquifer and consumption of dissolved oxygen. This study demonstrates that subaqueous nitrification of organic N in the aquifer, as opposed to in soils, might be a previously unrecognized source of NO3(-) to karst groundwater or other oxic groundwater systems.
许多喀斯特地区正经历着人口的快速增长和城市土地的扩张，与此同时，废水的产生量也在增加，硝酸盐（NO3(-)）对地表水和地下水的负荷模式也在发生变化。本研究以美国德克萨斯州中部的爱德华兹含水层系统为例，调查了区域喀斯特含水层系统中硝酸盐（NO3(-)）的变异性及其来源。本研究于 2008 年至 2012 年期间，在巴頓泉（Barton Springs）和圣安东尼奥（San Antonio）两个爱德华兹含水层段，采集了补给含水层的溪流、地下水井和泉水的样本，对氮（N）物种浓度和硝酸盐（NO3(-)）稳定同位素（δ(15)N 和 δ(18)O）进行了分析。此外，还利用了 1937 年至 2007 年收集的历史数据。NO3(-)浓度和排放数据表明，巴頓泉段地下水中 NO3(-)浓度的短期（天到月）变化受个别风暴和多年干湿循环的影响，而圣安东尼奥段地下水中没有短期变化则表明其主要受区域水流路径传输的控制。在这两个段，NO3(-)浓度的长期（年到十年）增加不能归因于水文条件；相反，同位素比值和土地利用变化表明，化粪池系统和经处理的废水土地应用可能是 NO3(-)负荷增加的来源。这些结果突出了城市废水对喀斯特含水层硝酸盐（NO3(-)）污染的脆弱性。2008 年至 2010 年，对巴頓泉段补给和排放的 N 物种负荷进行的分析表明，总氮存在整体质量平衡，但补给水中的有机氮浓度较高，NO3(-)浓度较低，与含水层内有机氮的硝化作用和溶解氧的消耗一致。本研究表明，与土壤相比，含水层中有机氮的水下硝化作用可能是硝酸盐（NO3(-)）进入喀斯特地下水或其他好氧地下水系统的一个以前未被认识到的来源。
