3.1 Historical Sediment and Metals Concentrations
Soil types in the Greater Houston Metropolitan Area (GHMA) are diverse (Figure S3-A in the SI). In general, the northern and northwestern parts of the GHMA are covered with sand, whereas the eastern and southern parts have more clay soil. Silt is the dominant grain type in areas closer to the main waterbodies and riverine/bayou system in the area. Almost all of the soils in the GHMA have a relatively high potential of erosion as shown in Figure S3-B in the SI. This high erosion potential combined with a history of flooding have led to significant loads of sediment into the GBES.
The data in Figure 2 illustrate flow exceedance rate curves in four of the major bayous in the GHMA for a 25-year period. The dots in Figure 2 represent the flow rates and their associated exceedance rates at the time when TSS samples were collected from the bayous over the years. As could be seen in the figure, the majority of the samples were collected when flow rates were low; an observation that is important because it highlights the lack of understanding of sediment loads associated with hurricanes and severe storms that disrupt the quasi-equilibrium that exists in sediment beds in natural water systems under normal flow regimes.