Toc l

A total of 15 wells were sampled, among them 2 were covered open wells, and the rest were tube wells. For open well sampling, grab sampling technique was used; for tube well sampling, water was sampled only after the existing water in the tube was completely replaced by a large volume of fresh water [15 (link)]. All samples were collected in the morning, when maximum water usage was reached for the local community [15 (link)]. Duplicate samples per well were collected. Samples were saved in clean and sterile polypropylene containers and transported to the laboratory on ice, and their characterization was done within 6 h of collection.
Total organic carbon (TOC) and total nitrogen (TN) in water samples were determined by TOC (TOC-L, Shimazu, Japan), NH₄⁺–N was determined by Nessler’s reagent spectrophotometry method, NO₃⁻–N was quantified by double wavelength ultraviolet spectroscopy method. Total phosphorus (TP) was quantified by potassium persulfate oxidation coupled with ammonium phosphomolybdate colorimetry. Total soluble iron (Fe) was examined by ascorbic acid reduction–phenanthroline spectrophotometric analysis method. Total plate counting (TPC) was conducted to numerate bacteria well water samples (data not shown due to the lack of significant correlation with ttds). The results of well water characterization were shown in Table 1.

Each of the seven soil samples was divided into three aliquots as replicates for MFCs operation. Before the operation, soil physiochemical properties of each aliquot were analyzed using routine methods (Page et al., 1982 ). Briefly, soil texture was determined by the sieve and pipette method. Soil maximum water holding capacity (MWHC) was determined by the difference between dry and soaked soil weights. Soil pH was measured at 1:2.5 (soil:water) and soil electrical conductivity (EC) at 1:5 (soil:water). Soil cation exchange capacity (CEC) was analyzed by the compulsive exchange method. Soil organic carbon (OC) was determined by K₂CrO₄ oxidation; total nitrogen (TN) by Kjeldahl digestion; and total phosphorus (TP) by colorimetry following NaOH digestion. Dissolved organic carbon (DOC; extracted by 0.5 M K₂SO₄) and humic carbon (HC; extracted by 0.1 M Na₄P₂O₇ and 0.1 M NaOH) were measured using a TOC analyzer (TOC-L, Shimadzu, Kyoto, Japan). Soil total dissolved iron (DFe_T) was extracted by 0.2 M H₂C₂O₄-(NH₄)₂C₂O₄ (McKeague and Day, 1966 (link)) and measured by flame atomic absorption spectroscopy (AA240, Agilent Technologies, Santa Clara, CA, USA). Soil microbial biomass carbon (MBC) was measured by the fumigation-extraction method (Vance et al., 1987 (link)).