Characteristics and source of microbial contamination of groundwater in Laolongdong basin

Laolongdong basin in Chongqing is a typical karst valley. The groundwater quality is closely linked to above-ground human activity. The study area has a high population density, with the rapid urbanization in the past few years. Sewage from many residents and factories flows through sinkholes into the underground aquifer. Meanwhile, the wastewater treatment and collection system has not been sufficiently developed,causing contamination of the underground river. This study indicates that livestock waste and urban sewage have resulted in high levels of microbial contamination. With social and economic development the groundwater contamination in Laolongdong basin was serious. Hydrogeological changes were monitored at 10 groundwater sampling points, and sewage samples were collected to evaluate their impact on Laolongdong groundwater and track the main sources of groundwater contamination. Taking into consideration the special hydrogeological background of the karst area, membrane filtration was employed and samples were analyzed for chemical composition and three kinds of bacteria used as indicators of the degree of fecal contamination in polluted groundwater. FC/FS was adapted to determine the source of microbial contamination. The results showed that, (1)The microbial contamination was not localized, the total bacteria count (2.3×102 to 5.5×104) and total coliform (0.8×104 to 0.9×106) exceeded the National Standard. The number of fecal coliform was 100 to 4.38×106 CFU?L-1. (2)The FC/FS ratio indicates that warm-blooded animals were the source of the microbes in samples 4# and 6#. Samples 1#, 7# and 8# were affected by urbanization. (3)There was high SO42-, PO43-, NO3- content. There was also close correlation among fecal indicator bacteria and SO42-, PO43-, NO3-. This indicates that urban sewage led to the serious microbial contamination and the main contamination source was human feces. Samples 2#, 3#, 5# and 10# were agriculture contamination points, therefore, the pollution was less serious (with NO3-).