Hydrochemical Characteristics, Controlling Factors, and High Nitrate Hazards of Shallow Groundwater in an Urban Area of Southwestern China

Abstract

Groundwater nitrate (NO₃^-) contamination has emerged as a critical global environmental issue, posing serious human health risks. This study systematically investigated the hydrochemical processes, sources of NO₃^- pollution, the impact of land use on NO₃^- pollution, and drinking water safety in an urban area of southwestern China. Thirty-one groundwater samples were collected and analyzed for major hydrochemical parameters and dual isotopic composition of NO₃^- (δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^-). The groundwater samples were characterized by neutral to slightly alkaline nature, and were dominated by the Ca-HCO₃ type. Hydrochemical analysis revealed that water-rock interactions, including carbonate dissolution, silicate weathering, and cation exchange, were the primary natural processes controlling hydrochemistry. Additionally, anthropogenic influences have significantly altered NO₃^- concentration. A total of 19.35% of the samples exceeded the Chinese guideline limit of 20 mg/L for NO₃^-. Isotopic evidence suggested that primary sources of NO₃^- in groundwater include NH₄⁺-based fertilizer, soil organic nitrogen, sewage, and manure. Spatial distribution maps indicated that the spatial distribution of NO₃^- concentration correlated strongly with land use types. Elevated NO₃^- levels were observed in areas dominated by agriculture and artificial surfaces, while lower concentrations were associated with grass-covered ridge areas. The unabsorbed NH₄⁺ from nitrogen fertilizer entered groundwater along with precipitation and irrigation water infiltration. The direct discharge of domestic sewage and improper disposal of livestock manure contributed substantially to NO₃^- pollution. The nitrogen fixation capacity of the grassland ecosystem led to a relatively low NO₃^- concentration in the ridge region. Despite elevated NO₃^- and F^- concentrations, the entropy weighted water quality index (EWQI) indicated that all groundwater samples were suitable for drinking. This study provides valuable insights into NO₃^- source identification and hydrochemical processes across varying land-use types.

Keywords: drinking water quality; groundwater; land use; nitrate; stable isotopes.

Figure 1

The location of the groundwater samples: (a) location of Chongqing City; (b) location of the study area in Chongqing City; (c) land use map of the study area and the location map of the groundwater samples.

Figure 2

Box plots of hydrochemical parameters in groundwater.

Figure 3

Piper diagram for hydrochemical types of groundwater.

Figure 4

Spatial distribution maps of hydrochemical components in groundwater: (a) K⁺; (b) Na⁺; (c) Ca²⁺; (d) Mg²⁺; (e) Cl⁻; (f) SO₄²⁻; (g) HCO₃⁻; (h) NO₃⁻; (i) F⁻; (j) pH; (k) TDS; (l) TH.

Figure 5

Gibbs diagrams of groundwater. (a) TDS vs. Na⁺/(Na⁺ + Ca²⁺); (b) TDS vs. Cl⁻/(Cl⁻ + HCO₃⁻).

Figure 6

Gaillardet diagrams of groundwater. (a) HCO₃⁻/Na⁺ vs. Ca²⁺/Na⁺; (b) Mg²⁺/Na⁺ vs. Ca²⁺/Na⁺.

Figure 7

Ion ratio plots and SI vs. TDS plots of groundwater. (a) Cl⁻ vs. Na⁺; (b) HCO₃⁻ vs. Ca²⁺; (c) SO₄²⁻ vs. Ca²⁺; (d) HCO₃⁻ + SO₄²⁻ vs. Ca²⁺ + Mg²⁺; (e) Mg²⁺ vs. Ca²⁺; (f) SI vs. TDS.

Figure 8

Cation exchange reaction plots of groundwater. (a) Ca²⁺ + Mg²⁺-SO₄²⁻-HCO₃⁻ vs. Na⁺ + K⁺-Cl⁻; (b) CAI-II vs. CAI-I.

Figure 9

Relationship plots of NO₃⁻, Cl⁻, and Na⁺ in groundwater: (a) NO₃⁻/Na⁺ vs. Cl⁻/Na⁺; (b) NO₃⁻/Cl⁻ vs. Cl⁻.

Figure 10

Cross plot of δ¹⁸O-NO₃ and δ¹⁵N-NO₃ in groundwater.

Figure 11

The land use map of the study area and the locations of groundwater samples with NO₃⁻ concentrations.

Figure 12

The drinking water quality of groundwater: (a) EWQI vs. TDS plot; (b) the spatial distribution map of EWQI in study area.