*4.2. Watershed Delineation*

This section introduces the watershed delineation results for the study area (Figure 4) (Table 2). The watershed delineation encompassed a comprehensive LIDAR elevation data reconditioning to well display the North and Central Watersheds' characteristics. Elevation reconditioning has revealed improved results in areas with very flat terrain. Previous studies had positive results with respect to their watershed delineation by performing this methodology [13]. Burning the waterways to the elevation data has enhanced the terrain to better support the current conditions of the elevation changes in the waterways. Generally, all the waterways within the area are man-made, which makes it challenging for the elevation data to capture the waterways. The north and central watersheds presented a total area of 3116 km<sup>2</sup> of which HWMD watershed presented an area of 1357 km<sup>2</sup> , RVD watershed is 1021 km<sup>2</sup> , and IBWCNF watershed is 737 km<sup>2</sup> (Table 2). HWMD watershed covers 68% of its area in Hidalgo County, 31% in Willacy County, and 1% in Cameron County. This watershed covers a wide central area of the LRGV region. It extends across nine cities in the region. Moreover, it covers the McAllen-Edinburg-Mission Metropolitan Statistical Area (MSA) of the LRGV region, which is ranked the 5th largest in the state of

Texas. The RVD watershed, located in the north area of the LRGV region, covers 30.7% in Hidalgo County, 68.9% in Willacy County, and 0.4% in Kennedy County. The city of Raymondville, San Perlita, and a northeast portion of the city of Edinburg are the only cities within the watershed. *Sustainability* **2021**, *13*, x FOR PEER REVIEW 11 of 23

**Figure 4.** North and Central subwatersheds within each of the watersheds; top: Raymondville Drain (RVD); middle: Hidalgo/Willacy Main Drain (HWMD); bottom: International Boundary & Water Commission North Floodway (IBWCNF). **Figure 4.** North and Central subwatersheds within each of the watersheds; top: Raymondville Drain (RVD); middle: Hidalgo/Willacy Main Drain (HWMD); bottom: International Boundary & Water Commission North Floodway (IBWCNF).

**Table 2.** Watershed delineation results for the three waterways. **Table 2.** Watershed delineation results for the three waterways.


*4.3. Nonpoint Sources*  In this section, the watershed sources that potentially contribute the most to NPS pollutants were identified. Table 3 shows the results of the ratio of PS and NPS pollution sources to the area of each watershed. The predominant land cover for the North and Central Watersheds is cultivated crops representing 53% of the total area located mostly in the northeast sector of the watersheds. This type of land use is within the downstream tributary areas of the watersheds. Urbanization areas within the North and Central Watersheds cover 13% of the total area. STLR were found near the coast of the three watersheds. **Table 3.** Ratio of NPS and PS pollution sources with respect to the area of each watershed. IBWCNF watershed is located 53% in Hidalgo County, 24% in Willacy County, and 24% in Cameron County. This watershed is within the southern area of the North and Central Watersheds and intersects with the Arroyo Colorado Watershed. Eight cities are included in the IBWCNF watershed. The IBWCNF branches off of the Main Floodway at the Llano Grande, a shallow lake located southwest of the city of Mercedes [57]. The IBWCNF Waterway is considered a man-made waterway approximately 77 km long and is used to divert the Arroyo Colorado's flow. The city of Mercedes is upstream of IBWCNF flow and downstream of the Arroyo Colorado Waterway when the flow is exceeding its capacity. During flood conditions, which the IBWC defines as flow exceeding 40 cubic meters per second, approximately 80% of the flow in the Arroyo Colorado is diverted to the IBWCNF [58].

#### **Sources HWMD RVD IBWNF**  *4.3. Nonpoint Sources*

Point Source Pollution

\* Quantified data.

Nonpoint Source Pollution Urbanized Areas 0.20 0.05 0.24 Cultivated Crops 0.47 0.52 0.59 STLR 0.06 0.20 0.04 Species \* 0.03 0.10 0.20 OSSFs 3.38 0.05 6.13 Colonias 0.25 0.01 0.29 Texas Land Application Permit 0.006 0.004 0.004 In this section, the watershed sources that potentially contribute the most to NPS pollutants were identified. Table 3 shows the results of the ratio of PS and NPS pollution sources to the area of each watershed. The predominant land cover for the North and Central Watersheds is cultivated crops representing 53% of the total area located mostly in the northeast sector of the watersheds. This type of land use is within the downstream tributary areas of the watersheds. Urbanization areas within the North and Central Watersheds cover 13% of the total area. STLR were found near the coast of the three watersheds.

About 73% of the HWMD watershed area is covered with NPS pollutants sources. The watershed's cultivated crops correspond to approximately 47%, and 20% of urbanized areas. Urban growth in the watershed will primarily occur in areas that are currently

Wastewater Outfalls 0.008 0.005 0.012 Municipal Solid Waste 0.013 0.004 0.004 MS4 Permit 0.006 0.001 0.016 Desalination Plants 0.001 0.001 0.003


**Table 3.** Ratio of NPS and PS pollution sources with respect to the area of each watershed.

\* Quantified data.

About 73% of the HWMD watershed area is covered with NPS pollutants sources. The watershed's cultivated crops correspond to approximately 47%, and 20% of urbanized areas. Urban growth in the watershed will primarily occur in areas that are currently cultivated and will influence the region's water quality [34]. Therefore, the HWMD watershed was identified with the highest ratio of urban areas among the other watersheds, with respect to their watershed area. The watershed encompasses 6.4% of STLR areas. Only El Suaz ranch pertains to the watershed. These STLR areas have grazing livestock activities that ultimately carry significant levels of bacteria. There were 46 species identified in this watershed along with two WMA units. La Palomas units, Longoria, and Fredrick, were found to possess hunting activities for their diversity of species. A total of 4591 OSSFs were found in the HWMD watershed from a total of 9170 in the north and central watersheds. All OSSFs have a potential for adverse environmental impact if they are improperly functioning, but those closer to streams present an elevated risk [34]. The watershed has 336 colonias, where 80 are classified with limited solid waste disposal, and 33 lack adequate solid waste and wastewater disposal. The total area of the colonias in the watershed is 26.8 km<sup>2</sup> .

NPS pollutants sources cover almost 86% of the total area of the RVD watershed. The watershed has 51% of cultivated crops and only 2% of urban areas. The RVD watershed encompasses 19% of STLR areas. King Ranch, East Foundation, and El Suaz are the ranches that cover the watershed. Not only agriculture activities take place within the STLR areas. Livestock also grazes in this area, which can increase the relative contribution of bacteria. Fecal pollution brought to the rivers through surface runoff and soil leaching represents the NPS pollution; its origin can be the wild animals and grazing livestock feces and cattle manure spread on cultivated areas [50–52]. A total of 56 OSSFs were identified in the watershed. The RVD watershed has only 13 colonias recorded from which 1 is limited to solid waste disposal and 3 lack of basic utilities. Colonias within the watershed cover an area of 21.6 km<sup>2</sup> .

The IBWCNF watershed corresponds to 73% of cultivated crops and 13% of urban areas. This watershed has the highest ratio of agricultural lands that can be a possible source of ammonia and nitrogen in the surface water. According to the EPA, watersheds could be affected by the level of decomposition of organic matters and some fertilizers used in agriculture. This watershed covers a portion of El Suaz ranch with 5% of STLR areas. There were 4523 OSSFs identified in this watershed, corresponding to a 6.33 ratio between the total OSSFs and the total area of the watersheds. The colonias cover an area of 23.4 km<sup>2</sup> within the IBWCNF watershed. This watershed has 216 colonias from which 65 lack proper solid waste disposal, and 51 lack both solid waste and wastewater disposal.

In summary, the HWMD watershed was identified with the highest ratio of urban areas among the other watersheds with respect to their watershed area. The identification of McAllen-Edinburg-Mission MSA in this watershed demonstrates the high presence of urban areas. The HWMD had 20.3% of urban areas and 8.8% from the three watersheds. In contrast, the IBWCNF presented a higher percentage of 24.3% in urban areas, but it only had 5.8% with respect to the overall area of the North and Central Watersheds. The RVD and IBWCNF watersheds were the ones to have greater NPS pollution derived from cultivated crops [46]. The RVD watershed was the highest with STLR areas.
