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Project: Assessment of Water Quality Concentrations and Loads from Natural Landscapes

Background and Objective

One of the challenges facing environmental managers who work to minimize pollutant loading to aquatic ecosystems is distinguishing the portion of “pollutants” (such as bacteria, trace metals and nutrients) that are attributable to natural sources. Data on the amount of these pollutants contributed from typical natural landscapes is lacking. Thus, managers are left without a scientific basis for differentiating water quality problems between natural and human causes. In order to promote more effective management measures, like Total Maximum Daily Loads (TMDLs), SCCWRP researched characteristics of water quality from natural landscapes, and then related these water quality concentrations to watershed properties.

The objective of this study was to evaluate properties of stream reaches in natural catchments throughout southern California, and to characterize water quality contributions from these catchments. Specific questions addressed by this study were: 1) What are the ranges of concentrations, loads, and fluxes of various metals, nutrients, solids, algae, and bacteria associated with storm and non-stormwater runoff from natural areas? 2) How do the ranges of constituent concentrations and loads associated with natural areas compare with those associated with urban (developed) areas and existing water quality standards? 3) How do the environmental characteristics of catchments influence constituent concentrations and loads from natural landscapes?


This three-year study took place from 2004 to 2007.


Twenty-two natural, open space study sites were selected that represented a range of conditions across six counties and twelve different watersheds. These were sampled during both wet and dry weather for a variety of water quality and physical parameters (e.g., solids, nutrients, metals and bacteria). The resultant data was analyzed in four main ways:

• To provide a baseline for expected water quality, ranges of natural concentrations were calculated for several water quality parameters.
• Data from natural sites were compared with previously collected data from developed sites to uncover significant differences.
• Data were compared to relevant water quality standards.
• Data from natural sites were analyzed to elicit the factors that contributed most to variability among sites.

Study sites in southern California coastal watersheds


The flux estimates generated from this study should be applicable to all coastal watersheds throughout southern California. Data may also be used to calibrate watershed computational models, which can serve as a valuable management tool. Major findings from this project were:

• Natural area water quality concentrations and loads were typically 1-2 orders of magnitude lower than those found in developed watersheds and dry and wet weather concentrations, loads, and fluxes from natural catchments had high variability;

• Wet-weather total suspended solids (TSS) levels in the natural catchments were similar to those in the developed catchments.

• Dry weather loading constituted a substantial portion of total annual load in some natural areas.

• Concentrations of several nutrients were higher than USEPA-proposed nutrient guidelines (i.e., US Environmental Protection Agency guidelines for Ecoregion III, 6). This finding indicates that background nutrient levels in southern California may be higher than in other portions of the country.

•  Peak pollutant concentrations and loads occurred later during storms in natural areas as compared to developed areas. Natural catchments did not appear to exhibit a "first flush" phenomenon during storms. However, storm duration was longer in natural catchments than in developed catchments, and the pollutograph was more spread out (i.e., relatively high concentrations persisted for longer).

• When comparing natural area concentrations to water quality standards, metals were normally below the guidelines, whereas bacteria and nutrients regularly exceeded guidelines.

• Watershed geology was the most influential factor creating variability in water quality across natural study areas. Those landscapes underlain by sedimentary rock generally produced higher concentrations of water quality parameters than those underlain by igneous rock.

• Other environmental factors such as watershed size, flow, rainfall, slope, canopy cover and land cover did not significantly affect water quality variability.


This project was conducted in collaboration with UCLA, the Los Angeles Regional Water Quality Control Board (LARWQCB) and the US Environmental Protection Agency (EPA) Region IX.


Assessment of Water Quality from Natural Landscapes - January 2010 presentation to SCCWRP member agencies describing the significance and results of the 2004 natural loadings study and a 2006 bacteria reference watershed study


Download data from this project
For more information on Assessment of Water Quality Concentrations and Loads from Natural Landscapes, contact Eric Stein at (714) 755-3233.
This page was last updated on: 6/25/2014