3. RESULTS AND DISCUSSION

3.1 water chemistry and nitrate concentration analysis

The field parameters and constituents of surface water and groundwater were summarized in Table 2. The DO values in surface water ranged from 1.21 to 14.72 mg/L with an average of 8.01 mg/L, indicated the existence of aerobic environment for surface water. The DO values in groundwater varied from 0.82 to 8.97 mg/L with a mean of 4.44 mg/L, suggested that both oxidizing and reducing environments were present in the groundwater. The NH4+ concentration in surface water ranged from 0.2 to 2.44 mg/L, with an average of 0.81 mg/L. The NH4+ concentration in groundwater ranged from 0.13 to 4.61 mg/L, with an average of 1.18 mg/L. 21 water samples exceeded the environmental quality standard for groundwater Class V (1.5 mg/L NH4+, GB/T 14848-2017) set by the Ministry of Ecology and Environment of China, and the higher NH4+concentration may be related to a large amount of turf cultivation and ammonia nitrogen fertilizer applied in the study area in recent years. The concentrations of DOC in surface water ranged from 2.2 to 30.72 mg/L, with an average of 11.98 mg/L; the concentrations of DOC in groundwater ranged from 1.74 to 35.16 mg/L, with an average of 14.49 mg/L.
Table 2 Statistical summary of chemical and isotopic parameters in SW and GW of the Huashan basin
The spatial distribution of nitrate in surface water and groundwater in the Huashan watershed are shown in Figure 2. The nitrate concentrations in surface water were all less than 1 mg/L in spring, and the nitrate concentrations in groundwater were all less than 2 mg/L. The concentration of NO3- in surface water in summer was less than 1 mg/L except at the basin outlet and upstream of the Xiyuan subbasin; The average NO3- concentration of groundwater was 2.26 mg/L, with four water samples having a concentration greater than 5 mg/L, located in the southeastern and southern regions of the watershed. The NO3- concentration was relatively low in the downstream area of the watershed. NO3- in surface water is higher in the downstream of the basin in autumn; the distribution of NO3- in groundwater in autumn is similar to that in summer, while the concentration is lower than that in summer. The concentration of NO3- in surface water further increased in winter, up to 5.1 mg/L in the upper reaches of the Xiyuan subbasin; the NO3- concentration in two groundwater samples exceeded 10 mg/L in winter, distributed near the lower reaches of the watershed.