Exploratory assessment of soil–water quality and microbial indicators across land-use conditions in a rural watershed of India

An exploratory assessment of soil quality, water quality, and microbial indicators was conducted at 15 independent paired sites in the rural watershed of Madavarayapuram, Tamil Nadu, India. At each site, 15 soil and 15 adjacent water samples were collected across agricultural, habitation, and fallow land-use settings. Physicochemical variables, culture-based microbial indicators, acid phosphatase activity, and phosphate-solubilizing activity were measured. Inverse Distance Weighting and Getis-Ord Gi* were used only for exploratory spatial visualization, whereas Spearman rank correlation and principal component analysis (PCA) were used as statistical analyses. The qPCR component was limited to four selected samples and is presented only as exploratory amplification evidence rather than as a watershed-wide quantitative estimate of bacterial abundance. Agricultural sites showed the highest observed water electrical conductivity, nitrate, and phosphate values, reaching 382 µS/cm, 7.37 µg/mL, and 0.34 µg/mL, respectively. Across the 15 paired sites, presumptive E. coli showed a moderate positive soil–water association (ρ = 0.64, p = 0.010), whereas total coliforms (ρ = 0.86, p < 0.001) and total heterotrophic bacteria (ρ = 0.89, p < 0.001) showed stronger positive associations. The first two PCA axes summarized 72.9% of the observed multivariable variation, while Global Moran’s I did not identify statistically significant global spatial autocorrelation for the measured variables. The main finding is that land-use-associated physicochemical conditions covaried with microbial indicators at the sampled locations, but the paired-site associations do not demonstrate directional microbial transfer or direct hydrological connectivity. Future work should use repeated seasonal sampling, source-stratified designs, direct hydrological measurements, and fully validated molecular assays.