These are calculations of eight non-point source pollutants that have been found in stormwater in five watersheds in Harris County. The calculations are based off a study published in 1991 for the Galveston Bay National Estuary Program. [42] Although the calculations are a bit dated, studies that measure the concentrations of non-point source pollutants in water are very expensive to conduct. Because of this, very few have been conducted in recent years. However, the figures from 1991 are still accurate enough to be used in 2016.[43]
In order to completely understand the amount of pollutants in the water, we created two scenarios based on the scenarios presented in the 1991 study.
Scenario 1: Annual rainfall data from 1996 to 2016 was collected from five rain gauges (one from each watershed) and averaged, excluding the highest value and the lowest value. Then, the average annual rainfall for each watershed was divided by 365 to represent the average daily rainfall.
Scenario 2: Using the same five rain gauges from Scenario 1, the highest daily rainfalls for the respective year from 1996 to 2016 were averaged. This is called the Storm Average and represents extreme weather events. Then, these averages were multiplied by an adjusting factor of 0.92 to reflect the distribution of rainwater over a broad area.
White Oak Bayou: 585: Vogel Creek @ Victory Drive [41]
Based on this rainfall data, it was assumed that the runoff values in 2016 were proportional to the runoff values from 1991. This is a simplified calculation that does not account for the fact that different surfaces (concrete, grass, soil, etc.) have different rates of runoff. The runoff values for Scenario 1 are much larger than the runoff values for Scenario 2. Because of this, the amounts of pollutants in Scenario 1 are much larger than the amount of pollutants in Scenario 2. Please note that pollutants are measured in different units; this is to make the numbers more understandable.
It was assumed that the concentrations of pollutants in 2016 were the same, or negligibly different, from the concentrations presented in the 1991 study. Based on this assumption and the adjusted runoff values, the loads were calculated by multiplying the concentration and the runoff volume.
Any amount of a non-point source pollutant in water decreases the water's overall quality.
Data for heavy metals was presented in the 1991 study, but the concentration was so small that it had no significant effect on overall water quality.