Dry-Weather Metals and Bacteria Loading in an Arid, Urban Watershed: Ballona Creek, California

Dry-season urban runoff from watersheds in arid regions can contribute substantial concentrations and loads of bacteria and metals to downstream receiving waters. However, there are few studies that have evaluated the relative contributions of different sources of these constituents along a specific creek or channel. This study involved analysis of the relative contribution of various storm drain sources to the total dry-season loading of metals and bacteria in the Ballona Creek (California) watershed. Approximately 40 actively flowing storm drains and 12 in-river sites were sampled three times during the spring and summer of 2003 for flow, total and dissolved metals, and bacteria. These data were analyzed in terms of mean concentration and load, temporal variability, and spatial distribution of substantial inputs to the creek. In general, Ballona Creek exhibited a bimodal distribution of elevated metals and bacteria, corresponding to locations where storm drains with consistently high concentrations and loads discharge to the creek. Of the 40 drains sampled, 4 accounted for 85% of the daily storm drain volume. Between 91% and 93% of the total daily load for metals was contributed by eight drains, while nine drains consistently had the highest concentrations of metals and bacteria. Metals concentrations were observed to vary by up to five-fold and bacterial counts by up to five orders of magnitude on an intra- and inter-annual basis. However, despite this variability, a relatively small number of storm drain inputs can be expected to account for the majority of loading.