Influence of Agricultural Activity and Aquifer Intrinsic Vulnerability on Groundwater Quality in the Dagu River Watershed (Qingdao, China)

Contamination of the shallow aquifer by elevated nitrate concentrations is a common problem in many rural regions of the world. An aquifer that has both aquifer intrinsic vulnerability and irrigated land uses is especially susceptible to this type of contamination. A study was carried out to evaluate an aquifer\´s intrinsic vulnerability and agricultural activity\´s impact on groundwater quality in the Dagu River in Qingdao, which is located in the eastern part of China. The Dagu River aquifer supplies 50%-60% of total water usage for Qingdao. Groundwater hydrochemistry was investigated. Regarding sampling, 436 samples in groundwater and river water were collected from April of 2000 to September of 2006. Results indicate the main forms of contamination are NO₃^--N, CaCO3, TDS, F^-, and Cl^- which exceed Chinese Groundwater Quality Standard III (GB/T14848-93) by 82%, 57%, 23%, 20%, and 11% respectively. Nitrate concentration is calculated as nitrogen concentration, i.e. NO₃^--N. In particular, nitrate contamination in the shallow aquifer was evaluated by comparing NO₃^--N concentration versus parameters affecting aquifer intrinsic vulnerability (i.e., depth to water table, impact of the vadose zone, etc.). Also, the effect of land use and fertilizer usage on NO₃^--N concentration in groundwater was analyzed. The results suggest intensive agricultural activities, especially an overabundance of fertilizer used in vegetable planting, is an important key factor for nitrate contamination. When much of the fertilizer nitrogen is not converted into harvested crops, it leaves a significant fraction available for leaching into the thin sandy vadose zone which is called the "infiltration window".