Development of an integrated hydrochemical index for delineating livestock manure-derived groundwater plumes in agro-livestock farming areas

To delineate a livestock manure-derived groundwater plume (LDGP) in agro-livestock farming areas with the extensive use of chemical fertilizers, multilevel monitoring wells (MLWs) were installed for depth-specific sampling of groundwater, and then hydrochemical, nitrate N-O isotopic and microbiological data of shallow groundwater collected from the MLWs were evaluated with the help of multivariate statistical tools. The LDGP was distinguished from the pervasive agricultural contamination based on δ¹⁵N_nitrate (10‰). Fecal coliforms and Escherichia coli were not observed in the LDGP, whereas bovine enterovirus type 2 was detected at a sample collected at a depth of 18 m below ground level (bgl) downgradient manure piles, indicating a potential virus risk. Among hydrochemical parameters, Ca²⁺, Mg²⁺, Na⁺, Cl^- and SiO_2(aq) were shown to be effective indicators to trace the LDGP. On the other hand, nitrate was not effective to discriminate the LDGP because of denitrification in deep parts (>6 m bgl) within the LDGP, although nitrate contamination was serious at shallow parts (≤6 m bgl; 62.4 to 119.1 mg/L, median 97.9 mg/L). Thus, an integrated hydrochemical index consisting of Ca²⁺, Mg²⁺, Na⁺, Cl^- and SiO_2(aq) was suggested based on the result of principal component analysis with isometric log-ratio transformed data to delineate the LDGP in shallow unconsolidated aquifers overlying silicate bedrocks, and a threshold of the index was determined to be 1.17. The application of the index to three other agro-livestock farming areas with prevalent agricultural contamination successfully distinguished the groundwater samples within the LDGP (11 of 40 samples) that showed the index values exceeding the threshold and a median δ¹⁵N_nitrate of 12.5 ‰. The study results show that the hydrochemical index can be used for the quick evaluation of a LDGP in agro-livestock farming areas given its cost efficiency and easily accessible analysis devices for water chemistry compared to isotopes, although the broad use of the index to determine LDGPs needs to be further verified by application to various geology and land uses. Livestock manures should be carefully managed to protect groundwater resources, including impermeable covers above or beneath the manure piles, given the potential risk of virus at deep parts and nitrate at shallow parts within the LDGP. The methodology to select hydrochemical indicators for distinguishing a LDGP and the combination of the indicators to develop an index suggested in this study will be useful to build new indices adapted to local conditions.