## 2 Data and Methods
### Chemical Analysis and Quality Assurance of the Data  
In order to comprehensively consider the water quality in the eastern coastal zone of Hainan Island, a total of 112 surface water and groundwater samples were taken in three batches from 11 to 13 August 2021, with reference to the \"1:50,000 Regional Hydrogeological Survey Specification\" and \"Surface Water and Wastewater Monitoring Specification\" for the point layout and sampling standards. An amount of 5 L of water samples were collected, and the water samples were filtered with a 0.45 \(\upmu\)m filter membrane immediately after collection and stored in a thermal chamber at 4 \({}^{\circ}\)C, protected from light, total phosphorus (TP), total nitrogen (TN), heavy metals (As, Hg, and Cr (VI)), for a total of nine.  
Among them, CODMn index refers to the \"determination of permanganate index GB/T 11892-1989\" [29], using a 50 mL buret for determination; the detection limit is 0.5 mg/IL; COD determination using the rapid closed catalytic digestion method; BOD5 determination using rapid closed catalytic digestion method, refer to the \"Water and wastewater monitoring and analysis methods (fourth edition) Supplement\" State Environmental Protection Administration (2002); NH4\({}^{+}\)-N using nano-reagent spectrophotometry (HJ 535-2009) [30]; TP determination using ammonium molybdate spectrophotometry; TN determination using alkaline potassium persulfate ablation--UV spectrophotometry; heavy metals (As, Hg, and Cr(VI)) using diphenylcarbonyl dihydrorazole spectrophotometry (Cr(VI)) and atomic fluorescence hair (As, Hg), test The results meet the quality requirements. These nine ions were tested and analyzed with the following information (using NH4\({}^{+}\) as an example):  
Principle of method: Ammonia nitrogen in the form of free ammonia or ammonia ions reacts with the nanoreagent to form a light reddish-brown complex. The absorbance of this complex is proportional to the ammonia-ammonia content. The absorbance of the complex  
Figure 1: Distribution of sampling sites in the Wanquan River.  
can be measured at a wavelength of 420 nm. Apparatus and equipment: (1) Visible spectrophotometer with a 20 mm cuvette; (2) ammonia distillation apparatus with a 500 mL distillation flask. Chemical samples used: \(MgO\); \(HCl\); \(HgCl_{2}-KI-KOH\); \(HgI_{2}-KI-NaOH\); \(KNaC_{4}H_{6}O_{6}4H_{2}O\); \(Na_{2}S_{3}O_{3}\); \(Na_{2}CO_{3}\); \(ZnSO_{4}\); \(7H_{2}O\); \(NaOH\); \(H_{3}BO_{3}\); \(NH_{4}Cl\); _bromthymol blue_. Calculated as:  
\[\rho_{N}=\frac{A_{s}-A_{h}-a}{b\times V} \tag{1}\]  
where \(\rho_{N}\) is the mass concentration of ammonia nitrogen in the water sample, calculated as nitrogen, mg/L; \(A_{s}\) is the absorbance of the water sample; \(A_{h}\) is the absorbance of the blank test; \(a\) is the intercept of the calibration curve; \(b\) is the slope of the curve; \(V\) is the volume of the test material, mL.  
Accuracy and precision: For the standard solution of 1.21 mg/L ammonia, the repeatability limit was 0.028 mg/L, the reproducibility limit was 0.075 mg/L, and the recoveries ranged from 94% to 104%. Quality assurance and quality control were carried out through the following measures: absorbance of the reagent blank should not exceed 0.030 (10 mm cuvette); preparation of Nessler; preparation of potassium sodium tartrate; flocculation and precipitation processes; pre-distillation of the water sample; cleaning of the distillator.