Total Loads and Water Quality in the Corpus Christi Bay System Presented by: Ann Quenzer and Dr. David Maidment Special Thanks: Corpus Christi Bay National Estuary Program Ferdinand Hellweger Dr. Nabil Eid Dr. George Ward Dr. Neal Armstrong

Purpose u To determine the rainfall/runoff relationship u To estimate the point and non-point source loads to the bay system u To quantify the relationship between the total loads and the bay system water quality

Basic Concept Point and Non-point Estimation Total Loads Routing Water Quality Calculate Flow and Total Loads Linkage of the Two Models Steady-State Model

Purpose u To determine the rainfall/runoff relationship

Watershed Delineation Sub-Watersheds

Precipitation + = Precipitation Trend over Bay System Merged Precipitation Files Oregon State University Precipitation Data

Regression Inputs and Outputs

Surface Water Runoff

Mean Annual Runoff (mm/yr) Land Use + Precipitation

Precipitation and Runoff Gradient Precipitation and Runoff Gradient from South (A) to North (B) along the Bay System Precipitation and Runoff Gradient Locations in the South (A) and North (B)

Runoff Into Each Bay System Middle Bay System 24.5 m 3 /s 34% of total flow Entire Bay System = 72 m 3 /s North Bay System 40.5 m 3 /s 56% of total flow South Bay System 7 m 3 /s 10% of total flow

Bay System Water Balance Entire Bay System

Bay System Water Balance North Bay System Middle Bay System South Bay System

Purpose u To estimate the point and non-point source loads to the bay system

Total Constituent Loading Land Surface Load Point Source Load Atmospheric Load ? Sediment Load ?

Land Surface Constituent Loading Load [Mass/Time] = Runoff [Vol/Time] x Concentration [Mass/Vol]

Land Use USGS Land Use (1970’s) Addition of Missing Land Use

Percent Land Use Total Study Area Legend

EMC Table

Point Sources Texas Natural Resources Conservation Commission (TNRCC) Water Quality Segmentation

Loads Routing

Elevation Grid Modification Methodology

Model Connection Methodology

Load Routing Methodology

Total Constituent Loads Input for Water Quality Model Connection of Both Models Bay Water Quality

Total Load to Bay System

Load Sources

Load Contribution

Atmospheric Contribution Total Nitrogen Atmospheric Load to Land Surface = 2,700 Kg/d which is 35% of Land Surface Load from agricultural land use. This calculation is made assuming the EMC of 4.4 mg/l for agriculture and a Nitrogen concentration of 1.1 mg/l in precipitation

Purpose u To quantify the relationship between the total loads and the bay system water quality

Bay System Segmentation Segmentation Used in the CCBNEP Project Clipped Segmentation from Drs. Armstrong and Ward

Bay System Model Methodology.

Water Quality Analysis Salinity Concentration and Mass Fluxes in Corpus Christi Bay. Fluxes Flow of water Advection Dispersion Loads Transport of Constituents Finite Segment Analysis

Observed vs. Expected Total Phosphorus (mg/l) Total Nitrogen (mg/l)

Observed vs. Expected Oil and Grease (mg/l)Copper (µg/l)

Observed vs. Expected Zinc (µg/l)Chromium (µg/l)

T-Test Results

Decay Rates (using three segment model)

Provisional EMC Data Original EMC Values from USGS Study (Baird, 1996) Total Nitrogen= 4.40 mg/l Total Phosphorus= 1.30 mg/l Provisional EMC Values from Agricultural Runoff Studies at King Ranch and Agricultural Field Near Edroy, TX (Mean Values Obtained from Okerman of USGS) Total Nitrogen= 1.49 mg/l Total Phosphorus= 0.47 mg/l

Provisional EMC Values Total Nitrogen and Total Phosphorus from Land Surface Sources to Bay System Using Original and Provisional EMC Values. Total Nitrogen Reduction = 54% Total Phosphorus Reduction = 60%

Provisional EMC Values Total Nitrogen and Total Phosphorus from All Sources to Bay System Using Original and Provisional EMC Values. Total Nitrogen Reduction = 27% Total Phosphorus Reduction = 38%

Provisional EMC Values

Conclusions u Strong South-North gradient in runoff from the land surface u Nearly all water evaporates from bays, little exchange with the Gulf u Nonpoint sources are main loading source for most constituents u Nitrogen, phosphorus, oil & grease loads are consistent with observed concentrations in the bays u Metals loads from land account for only a small part of observed concentrations in bays - a reservoir of metals in the bay sediments?