Spatial-temporal distribution, genesis and environmental impact of the shallow groundwater pH values in the Fangchenggang, Guangxi

This paper is the result of hydrogeological survey engineering.

Objective The purpose of this study is to uncover the spatial-temporal distribution, genesis, and environmental impacts of shallow acidic groundwater in the Fangchenggang area.

Methods In-situ pH values of shallow groundwater were determined in the study area during 2013 to 2015, including 214 samples in the dry season and 168 samples in the wet season. Spatial and temporal distribution, genesis, and ecological influences of the pH values were discussed.

Results Results show that the acidic groundwater samples (pH < 6.5) account for 79.3% and 64.3%, respectively, in the wet and dry seasons. In general, pH values in the wet season are lower than in the dry season. Values are generally lower in the low mountain areas (pH 5.0-5.5, mean 5.18, n=4) relative to the hillys (pH mostly 5.0- 6.5, mean 5.97, n=202), and the plain areas (pH typically 6.5- 8.5, mean 6.77, n=8).

Conclusions Occurrence of acidic shallow groundwater can be contributed primarily to the acid rain, and partly to the acidic unsaturated zone, and oxidation of sulfide minerals. Accordingly, seasonal changes of acidic rain should principally responsible for the spatial variations of groundwater pHs. By contrast, however, tides could increase groundwater pH values in the Jiangping area. Acidic groundwater threatens safe drinking water supply, because only 20.6% samples in the dry season having pHs within the drinking water standard thresholds. Also, it may promote release of some harmful elements from aquifer sediments, and erode underground pipe networks and building pile foundations. In addition, due to discharge, acidic groundwater could influence surface water environment and surface ecosystems. This study contributes to water resources management in the study area.