Spatial Variability of Phosphate Groundwater Based on Land Use - Land Cover and Groundwater Quality on Increasing Rural to Urban Areas

Phosphate pollution in groundwater represents a significant global environmental challenge, particularly in regions experiencing rapid urban expansion and intensified agricultural activities. This study addresses this issue by investigating the spatial variation of phosphate concentrations in groundwater within the Surabaya-Lamongan Groundwater Basin (SLGB) in East Java, Indonesia. The research aimed to assess the spatial distribution of phosphate in unconfined aquifers across various land use-land cover (LULC) categories, identify potential sources of contamination, and determine whether phosphate concentrations exceed the 0.2 mg/L threshold for potable water. A total of fifty-eight groundwater samples were systematically collected using a grid-based sampling method and analyzed with a UV-VIS spectrophotometer employing the phosphomolybdenum blue method. LULC classification was conducted using Landsat 7 ETM+ (2000) and Landsat 8 OLI TIRS (2021) imagery. The findings indicated that phosphate levels were significantly higher in shallow wells compared to deep wells (p < 0.05). Residential areas exhibited the highest average phosphate concentration (1.107 mg/L), followed by agricultural land (0.337 mg/L), both surpassing the drinking water standard. Industrial areas demonstrated the lowest concentration (0.168 mg/L). Significant spatial differences were observed among administrative regions (p < 0.05), with Gresik recording the highest average concentration (1.201 mg/L). A one-sample t-test confirmed that the overall average phosphate concentration (0.627 mg/L) significantly exceeded the permissible limit (p < 0.001). These results suggest that anthropogenic activities, particularly in residential and agricultural zones, are major contributors to phosphate contamination. Immediate groundwater protection measures are essential to prevent further degradation of water quality in the SLGB.