*4.5. Water Quality Parameters*

The water quality parameters samples for the north and central watersheds are shown in Figure 6, where the red line represents the screening level according to TCEQ water quality standards. HWMD watershed has *E. coli* levels higher than the screening level of 126 MPN/100 mL from 2017 and 2019 [2]. In 2019, the *E. coli* levels were above 2000 MPN/100 mL. The existence of high levels of bacteria is caused by a variety of NPS and PS pollution sources such as urban runoff, agricultural lands, ranches, WWO, OSSF, MS4s, and colonias. Ammonia levels in this watershed were below the screening level with 2.7 mg/L as N, which is considered the highest record. In 2018, the TKN levels were the highest compared to the other years with more than 3.0 MGL as N. The presence of TKN in the HWMD watershed, according to the EPA, can be traced to failing septic systems, croplands, and industrial discharges [59]. TP levels barely exceed the screening level of 0.7 mg/L with the maximum value of 0.8 mg/L in 2017. Moreover, the nitrite and nitrate levels found in the watershed are higher than the screening level of 1.95 mg/L [2,60]. Chlorophyll-a levels identified surpassed the screening level of 14 µg/L for the three years [2]. In 2018, chlorophyll-a had the highest level of 98 µg/L.

The RVD watershed had higher levels of *E. coli* compared to the other watersheds, which suggests that there could be several sources of NPS and PS such as septic tanks that can be leaking. Further, sewage may overflow from poorly structured sewage systems and create polluted stormwater runoff [61]. However, ammonia levels for the RVD watershed are acceptable since they are below the screening level of 0.33 mg/L with a maximum value of 0.2 mg/L in 2018 and 2019 [60]. The TKN levels mainly surpassed the screening level of 1.0 mg/L in 2018 and 2019. TP levels were lower in all the years recorded, with a maximum value of 0.4 mg/L in 2019. According to the USGS report, bank erosion is the main source of total phosphorus during flooding events that can be the potential source in these watersheds [62]. Nitrite and nitrate levels surpassed only in 2017, but the highest level identified was almost 6 mg/L as N in 2019. For Chlorophyll-a levels, the RVD watershed showed its highest level of 70 µg/L in 2019.

In the IBWCNF watershed, the levels of bacteria were identified to be higher in 2013, 2014, 2015, and 2019. The highest level was around 8000 MPN/100 mL in 2013. The bacteria levels from 2016 through 2018 were determined to be slightly below the screening level of 126 MPN/100 mL. The results showed, according to Olmstead [46], that the watershed is affected by wildlife with small contributions of domestic animals and point sources. The ammonia levels were identified to be less than the screening level during all the years. This finding indicates that the watershed is limited to carrying significant levels of ammonia from agricultural runoff. TKN levels have shown to be relatively higher than the screening level with the highest of 2 mg/L as N in 2018. High levels of total nitrogen are caused by the decomposition of detritus and any anthropogenic loadings [63]. High levels of total nitrogen are caused by the decomposition of detritus and any anthropogenic loadings [63]. TP levels were lower than the screening level of 0.7. The IBWNF watershed is limited to algae growth since TP levels are low. Nitrite and nitrate levels are higher than the screening levels; 7 mg/L was the highest level recorded in 2015. Chlorophyll-a levels were determined to be higher than the screening levels for nearly all the years. This finding indicates the presence of excess quantities of algae [64].

**Figure 6.** Comparison between water quality concentration levels for Hidalgo/Willacy Main Drain (HWMD), Raymondville Drain (RVD), and International Boundary & Water Commission North Floodway (IBWCNF)): (**a**) *E.coli*, (**b**) total nitrogen, (**c**) nitrate + nitrite, (**d**) chlorophyll-a, (**e**) ammonia, (**f**) total phosphorus, and (**g**) organic nitrogen. Redline represents the TCEQ screening level for each parameter (data source: SWQMIS). **Figure 6.** Comparison between water quality concentration levels for Hidalgo/Willacy Main Drain (HWMD), Raymondville Drain (RVD), and International Boundary & Water Commission North Floodway (IBWCNF)): (**a**) *E.coli*, (**b**) total nitrogen, (**c**) nitrate + nitrite, (**d**) chlorophyll-a, (**e**) ammonia, (**f**) total phosphorus, and (**g**) organic nitrogen. Redline represents the TCEQ screening level for each parameter (data source: SWQMIS).
