Unraveling the drivers of fecal indicator bacteria dynamics in karst spring hyporheic zones: The role of external loading, hydrodynamics, and environmental forcing
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Abstract: Fecal Indicator Bacteria (FIB) are widely used to assess microbial safety in drinking water sources; however, their dynamics in karst springs hydraulically connected to rivers remain insufficiently understood due to strong hydrodynamic disturbances. This study investigated a large karst spring and its confluent river in South China to examine FIB contamination and the controls on FIB spatiotemporal patterns in the spring outlet hyporheic zone. Most hyporheic-zone and aquifer samples exceeded the relevant limits for Total Coliforms (TC) specified in the Chinese Standard for Groundwater Quality (GB/T 14848–2017) and Standards for Drinking Water Quality (GB 5749–2006), and Fecal Coliforms (FC) were detected in nearly half of the samples. During non-backflow periods, FIB abundances were significantly higher in summer and autumn than in spring and winter. Spatially, FIB generally increased downstream within the hyporheic zone, with high levels confined downstream at low water levels but extending upstream at medium and high water levels. Potential contamination sources included river backflow, aquifer discharge, sewage leakage, and rainfall-induced runoff. FIB were not clearly related to most physicochemical parameters, but were associated with organic matter and water temperature. River backflow strongly influenced FIB dynamics by altering water exchange and the physicochemical environment. During the two observed river backflow events, strong backflow mainly caused sharp FIB increases in the hyporheic zone and aquifer through intensified hydrodynamic transport and particle-associated migration, whereas weak backflow, despite not reaching the aquifer, still increased FIB within the hyporheic zone under reduced dilution and favorable nutritional and microecological conditions. High FIB concentrations and detection frequencies, together with elevated chemical indicators, indicate poorer microbial and chemical water quality in the hyporheic zone. These findings clarify the factors and processes shaping FIB dynamics in karst spring hyporheic zones and support drinking water source protection and microbial water quality management in vulnerable karst regions.
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Key words:
- Hyporheic zone /
- Fecal indicator bacteria /
- Hydrodynamics /
- Drinking water safety /
- Water environment
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Figure 1. Study area
Notes: (a) Geographic location of the study area in Guangxi, China. (b) Schematic plan of LingShui Spring and adjacent Wuming River, showing the locations of three river sampling sites (MXY, MXDZ, and LB) and surrounding residential areas. (c) Aerial view of the LingShui Spring area, illustrating the sampling points from aquifer (Y1, Y2), spring outlets (S1-S3), lake water (L1-L6), and their spatial distribution within the spring system. (d) Spring outlet (S2) on May 31, 2021 (before backflow). (e) Spring outlet (S2) on June 2, 2021 (during the flooding). (f) Spring outlet (S2) on September 6, 2024 (before backflow). (g) Spring outlet (S2) on September 8, 2024 (during the flooding).
Figure 2. Spatial variations in TC (a) and FC (b) during non-backflow periods, and their concentrations in the aquifer (c).
Notes: Sampling locations are categorized along the hydrological flow path from the spring outlet (SP: S1–S3) to the central lake (ML: L1–L5), and then to the downstream lake (DL: L6). Log10 (TC+1) and Log10 (FC+1) represent the base-10 logarithmic transformation of TC and FC concentrations, respectively, with 1 added to avoid undefined values when concentrations are zero.
Figure 4. TC (a) and FC (b) count at the spring outlet (S2) and downstream lake (L6) under non-backflow and backflow conditions.
Notes: The area to the left of the dashed line represents the non-backflow period, and the area to the right represents the backflow period during the 2024 backflow event.
Figure 6. Variations in physicochemical parameters in response to lake water level fluctuations during backflow events.
Notes: (a) Water Temperature (WT) and Specific Electrical Conductivity (SEC) during the 2024 backflow event. (b) pH and Dissolved Oxygen (DO) during the 2024 backflow event. (c) Turbidity (Turb.), Specific Electrical Conductivity (SEC), and Water Temperature (WT) during the 2021 backflow event.
Figure 9. Conceptual model of the FIB sources and dynamics in the karst spring outlet hyporheic zone under contrasting hydrodynamic conditions.
Notes: The model illustrates FIB dynamics under three hydrodynamic scenarios: (a) the non-backflow period, (b) a strong backflow event, and (c) a weak backflow event. FIB mainly originated from the aquifer, river backflow, surrounding domestic sewage, and rainfall-induced runoff. During the non-backflow period, FIB concentrations were higher in summer and autumn than in spring and winter. Generally, they increased from the spring outlet to the central lake, lake tail, and river-affected zone. Both river backflow events led to marked increases in FIB concentrations at LS. The strong backflow event was associated with an approximately 8 m rise in water level, with effects extending to both the hyporheic zone and the connected aquifer. In contrast, the weak backflow event involved a water-level rise of only approximately 2.5 m, and its influence was mainly restricted to the hyporheic zone.
Table 1. Seasonal variation of integrated environmental factors in the hyporheic zone (mean ± SD).
Spring (N=17) Summer (N=19) Autumn (N=23) Winter (N=23) pH 7.32±0.13 7.18±0.10 7.23±0.18 7.38±0.17 WT (°C) 24.21±0.39 24.46±0.23 24.40±0.23 24.01±0.34 DO (mg/L) 4.31±0.54 4.06±0.75 3.88±0.51 5.11±1.13 SEC (μS/cm) 378.9±15.1 380.4±18.2 383.6±16.0 381.4±14.2 Chl-a (μg/L) 0.22±0.20 0.18±0.16 0.20±0.30 0.09±0.08 CDOM (μg/L) 0.02±0.07 0.81±1.13 0.84±0.90 0.15±0.23 ORP (mV) 331.6±13.7 332.4±85.6 336.3±43.0 339.5±44.2 Table 2. Measured physicochemical parameters in hyporheic zone and river.
Spring outlet Lake River Range Mean Range Mean Range Mean pH 6.82–7.45 7.24 7.01–7.93 7.31 7.35–8.43 7.75 WT (°C) 23.93–24.72 24.27 23.53–24.95 24.27 14.71–31.32 24.85 DO (mg/L) 3.10–7.44 4.13 3.38–7.15 4.52 4.35–9.12 7.32 SEC (μS/cm) 336.3–413.9 378.51 363.8–398.2 383.02 222.7–414 294.5 Chl-a (μg/L) 0.00–0.91 0.11 0.03–0.91 0.21 0.55–9.75 2.15 CDOM (μg/L) 0.00–2.31 0.25 0.00–4.46 0.67 4.26–19.86 11.19 DOC (mg/L) 0.25–4 1.07 0.23–5.06 1.31 1.05–15.76 7.77 Turb. (NTU) 0.01–0.86 0.43 0.01–1.38 0.57 3.36–11.40 7.62 Cl− (mg/L) 5.09–6.89 6.14 5.00–6.77 5.94 6.04–33.96 9.92 SO42− (mg/L) 0.96–8.54 4.09 1.88–5.48 3.81 6.01–15.42 10.07 PO43− (mg/L) 0–0.12 0.05 0–0.08 0.03 0–0.64 0.101 NO3− (mg/L) 0–23.56 9.10 0–11.68 6.50 0–13.14 2.77 NO2− (mg/L) 0–0.018 0.005 0–4.82 0.788 0–3.18 0.439 NH4+ (mg/L) 0–0.02 0.005 0–0.062 0.011 0–0.2 0.034 Table 3. Changes in major pollutants in the hyporheic zone before and after river backflow
Period Date Site Cl− (mg/L) SO42− (mg/L) NH4+ (mg/L) NO2− (mg/L) NO3− (mg/L) PO43− (mg/L) Normal period 2021/1/13 L6 5.84 5.48 – – 8.94 – 2021/4/7 L6 5.96 3.36 0 0.003 8.94 0.08 Backflow period 2021/6/2 12:00 Y2 6.54 7.49 0 3.54 0.36 0 2021/6/3 8:00 Y2 6.54 9.68 1.35 0.003 9.55 0.08 2021/6/4 10:30 Y2 6.30 6.05 0 5.62 0 0 Normal period 2021/9/16 L6 5.00 2.8 – – – – 2021/12/22 L6 5.57 3.77 – – 9.92 0.05 2022/4/19 L6 6.40 3.54 0 3.57 – 0.03 2023/5/31 L6 6.68 4.01 0 4.82 0.80 0 2023/9/6 L6 6.37 3.25 0 0 – 0 2024/7/15 L6 6.26 3.54 0.02 0 10.49 0 Backflow period 2024/9/8 7:09 L6 8.18 11.42 0.08 0 0 0 2024/9/8 9:00 L6 7.83 4.22 0.12 0 0 0 2024/9/8 14:00 L6 10.10 15.46 1.27 0 0 0.071 2024/9/8 22:00 L6 8.07 10.66 0 0 0 0 2024/9/8 22:00 S2 7.69 10.74 0 0 0 0 Notes: – indicates missing data Table 4. Correlation coefficients for environmental factors with FIB.
Log10 (TC +1) Log10 (FC + 1) Log10 (NO3− + 1) −0.579** −0.439** Log10 (NO2− + 1) 0.409* 0.432* Log10 (NH4+ + 1) −0.007 −0.311 Log10 (PO43− + 1) −0.308 0.003 Log10 (Turb. + 1) 0.436 0.157 Log10 (DOC + 1) 0.392* 0.274 Log10 (Chla + 1) 0.329** 0.453** Log10 (CDOM + 1) 0.418** 0.314* Log10 (WT + 1) 0.423** 0.202 Log10 (pH + 1) 0.142 0.096 Log10 (ORP + 1) 0.130 0.073 Log10 (SEC + 1) −0.043 0.100 Log10 (DO + 1) 0.002 −0.051 Notes: Values represent Pearson correlation coefficients. "**" indicates significance at p < 0.01; "*" indicates significance at p < 0.05. Log10 (TC + 1) and Log10 (FC + 1) represent the base-10 logarithmic transformation of TC and FC concentrations, respectively, with 1 added to avoid undefined values when concentrations are zero. -
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