## 3 Results and Discussion
### Heavy Metal Assessment through Various Indices  
An investigation of surface water quality through the analysis of the Nemerow pollution index was conducted on a tributary in Nanjing in September (Figure 3). The indices showcased a fluctuating range from 0.5309 to 0.7988. Three sample points (Samples 8-10)  
Figure 3: Nemerow pollution index in the surface waters of the tributary. illustrated indices surpassing 0.7, inferring a minor pollution level at these locations. The scenario implies a possibility of specific pollution sources or events near or upstream of these points. The diverse pollution levels within the tributary could emanate from various sources, given its exposure to agricultural, residential, and anthropogenic activities. Elevated indices, especially in Samples 8-10, might be attributed to agricultural runoff, a common concern in surface water pollution, especially following rainfall events, due to the washing away of fertilizers and pesticides into water bodies [49]. September's climate context might be pivotal, as late summer and early autumn can introduce climatic variables influencing runoff and pollution levels. Residential communities adjacent to the tributary can also significantly contribute to the pollution indices observed. Domestic waste, sewage mismanagement, and everyday activities can directly or indirectly introduce contaminants into the water body. Anthropogenic activities, such as industrial operations or recreational uses, might also adversely impact the water quality [50]. Illegal discharges, inadequate waste management, or non-compliance with environmental regulations exacerbate pollution issues in urban water bodies. Given the multifaceted potential sources, managing and mitigating pollution in the tributary necessitates an integrated approach. Employing biological remediation, enforcing stringent regulations on pollutant discharges, and community awareness programs might reduce current pollution levels and safeguard the tributary against future pollution threats. All sediment sampled locations exhibited PLI values considerably exceeding 1.0 and reaching values as high as 3.04, indicating the existence of concerning levels of pollution across all tested sites (Figure 4). The range of PLI from 2.16 (Sample 2) to a strikingly elevated 3.05 (Sample 10) confirms a ubiquitous contamination presence and intriguingly suggests a gradient or potentially variable sources of contaminative input across the sampled sites. Dissecting the variability and progressive increment in PLI values from Sample 1 to Sample 10 could be emblematic of a spatial relation to contamination sources. Considering the observable trend, we believe that the potential contaminative activities--such as agricultural runoffs and domestic waste outputs--could be the reason for these elevated PLI values. PLI values in this tributary were as high as three, indicating a high level of environmental quality degradation and potential risk to aquatic organisms [51; 52]. Elevated metal contents in sediments could result in bioaccumulation within aquatic organisms, thus infiltrating the food chain, consequently impacting higher trophic levels, and potentially posing threats to human health [53]. The observed escalation in PLI warrants a profound exploration into the bioavailability of these contaminants and potential bioaccumulation within local aquatic biota. Figure 4: Nemerow pollution index among sediment samples of the tributary. The consistently elevated Igeo values for specific heavy metals, particularly Arsenic and Cadmium, across various sediment samples indicate a pervasive contamination issue (Table 4). These findings are comparable to national and global contexts, where anthropogenic activities, notably agricultural runoff, anthropogenic activities, domestic discharge, and industrial emissions, have notoriously facilitated the percolation of heavy metals into aquatic ecosystems [1, 54, 55].