Microbial atrazine breakdown in a karst groundwater system and its effect on ecosystem energetics.

In the absence of sunlight energy, microbial community survival in subterranean aquifers depends on integrated mechanisms of energy and nutrient scavenging. Because karst aquifers are particularly sensitive to agricultural land use impacts due to rapid and direct hydrologic connections for pollutants to enter the groundwater, we examined the fate of an exogenous pesticide (atrazine) into such an aquifer and its impact on microbial ecosystem function. Atrazine and its degradation product deethylatrazine (DEA) were detected in a fast-flowing karst aquifer underlying atrazine-impacted agricultural land. By establishing microbial cultures with sediments from a cave conduit within this aquifer, we observed two distinct pathways of microbial atrazine degradation: (i) in cave sediments previously affected by atrazine, apparent surface-derived catabolic genes allowed the microbial communities to rapidly degrade atrazine via hydroxyatrazine, to cyanuric acid, and (ii) in low-impact sediments not previously exposed to this pesticide, atrazine was also degraded by microbial activity at a much slower rate, with DEA as the primary degradation product. In sediments from both locations, atrazine affected nitrogen cycling by altering the abundance of nitrogen dissimulatory species able to use nitrogenous compounds for energy. The sum of these effects was that the presence of atrazine altered the natural microbial processes in these cave sediments, leading to an accumulation of nitrate. Such changes in microbial ecosystem dynamics can alter the ability of DEA to serve as a proxy for atrazine contamination and can negatively affect ecosystem health and water quality in karst aquifers.

Natural and anthropogenic factors affecting the groundwater quality in the Nandong karst underground river system in Yunan, China.

The Nandong Underground River System (NURS) is located in a typical karst agriculture dominated area in the southeast Yunnan Province, China. Groundwater plays an important role for social and economical development in the area. However, with the rapid increase in population and expansion of farm land, groundwater quality has degraded. 42 groundwater samples collected from springs in the NURS showed great variation of chemical compositions across the study basin. With increased anthropogenic contamination in the area, the groundwater chemistry has changed from the typical Ca-HCO(3) or Ca (Mg)-HCO(3) type in karst groundwater to the Ca-Cl (+NO(3)) or Ca (Mg)-Cl (+NO(3)), and Ca-Cl (+NO(3)+SO(4)) or Ca (Mg)-Cl (+NO(3)+SO(4)) type, indicating increases in NO(3)(-), Cl(-) and SO(4)(2-) concentrations that were caused most likely by human activities in the region. This study implemented the R-mode factor analysis to investigate the chemical characteristics of groundwater and to distinguish the natural and anthropogenic processes affecting groundwater quality in the system. The R-mode factor analysis together with geology and land uses revealed that: (a) contamination from human activities such as sewage effluents and agricultural fertilizers; (b) water-rock interaction in the limestone-dominated system; and (c) water-rock interaction in the dolomite-dominated system were the three major factors contributing to groundwater quality. Natural dissolution of carbonate rock (water-rock interaction) was the primary source of Ca(2+) and HCO(3)(-) in groundwater, water-rock interaction in dolomite-dominated system resulted in higher Mg(2+) in the groundwater, and human activities were likely others sources. Sewage effluents and fertilizers could be the main contributor of Cl(-), NO(3)(-), SO(4)(2-), Na(+) and K(+) to the groundwater system in the area. This study suggested that both natural and anthropogenic processes contributed to chemical composition of groundwater in the NURS, human activities played the most important role, however.