Groundwater-surface water interactions and flux of organic matter and nutrients in an urban, Mediterranean stream.

The chemical quality of dissolved organic matter (DOM) and the speciation of nitrogen exported from urban catchments is of great importance to biogeochemical cycling in riverine and coastal receiving waters. Many urban streams in Mediterranean climates have a flashy hydrologic regime, which would suggest a rapid pulsing and shunting of solutes downstream. However, the role of these systems both as passive pipes for solute transport or as reactors for DOM and nutrient transformation is still an open question for urban, Mediterranean streams. To address this question, we evaluated changes in concentrations of inorganic and organic solutes and DOM optical properties in Alvarado Creek, a perennially-flowing, urban, first-order tributary of the San Diego River in San Diego, CA, USA, during dry weather (baseflow) conditions and during four storm events in 2016-2018. Chloride and sulfate concentrations corroborate the supposed saline groundwater supply that maintains perennial flow and brackish nature in this urban stream. During dry weather, high proportions of protein-like fluorescent component (AC4) and downstream decreases in total dissolved nitrogen (TDN) and nitrate imply in-stream processing (nitrification and denitrification). By contrast, storm hysteresis curves indicate that the supply of DOM and TDN was not exhausted over the duration of a storm event, whereas nitrate was eventually depleted, presumably because nitrification could not keep up with the export of nitrate from source areas. Rapid decreases in chloride during the storm hydrograph coincided with a shift in specific ultraviolet absorbance (SUVA) and fluorescence index (FI) to more terrestrially-derived and aromatic carbon sources, most likely from interflow of stormwater through vadose zone soils. On an annual basis, the export of microbially-derived DOM during dry weather was higher than the export of terrestrially-derived DOM during storm events; both represent important carbon inputs to coastal waters.

An Assessment of Ambient Water Quality and Challenges with Access to Water and Sanitation Services for Individuals Experiencing Homelessness in Riverine Encampments.

Individuals experiencing unsheltered homelessness face significant barriers to accessing water, sanitation, and hygiene services, but the risks associated with this lack of access and barriers to service provision have been largely understudied. We analyzed water samples upstream and downstream of three homeless encampments in the San Diego River watershed and interviewed service providers from public and nonprofit sectors to assess local perceptions about challenges and potential solutions for water and sanitation service provision in this context. Water upstream from encampments contained detectable levels of caffeine and sucralose. Escherichia coli concentrations downstream of the encampments were significantly greater than concentrations upstream, but there was no significant change in the concentrations of other pollutants, including caffeine and sucralose. The HF183 marker of Bacteroides was only detected in one sample upstream of an encampment and was not detected downstream. Overall, there was insufficient evidence to suggest that the encampments studied here were responsible for contributing pollution to the river. Nevertheless, the presence of caffeine, sucralose, and HF183 indicated that there are anthropogenic sources of contamination in the river during dry weather and potential risks associated with the use of this water by encampment residents. Interviews with service providers revealed perceptions that the provision of water and sanitation services for this population would be prohibitively expensive. Interviewees also reported perceptions that most riverbank residents avoided contact with service providers, which may present challenges for the provision of water and sanitation service unless trust is first built between service providers and residents of riverine encampments.

Fluorescence-based monitoring of anthropogenic pollutant inputs to an urban stream in Southern California, USA.

Fluorescence spectroscopy has been increasingly used to detect sewage and other anthropogenic contaminants in surface waters. Despite progress in successfully detecting bacterial and sewage inputs to rivers over diverse spatial scales, the use of fluorescence-based in-situ sensors to track contaminant inputs during storm events and to discern bacterial contamination from background natural organic matter (NOM) fluorescence have received less attention. A portable, submersible fluorometer equipped with tryptophan (TRP)-like and humic-like fluorescence sensors was used to track inputs of untreated wastewater added to natural creek water in a laboratory sewage spill simulation. Significant, positive correlations were observed between TRP fluorescence, the TRP:humic ratio, percent wastewater, and Escherichia coli concentrations, indicating that both the TRP sensor and the TRP:humic ratio tracked wastewater inputs against the background creek water DOM fluorescence. The portable fluorometer was subsequently deployed in an urban creek during a storm in 2018. The peak in TRP fluorescence was found to increase with the rising limb of the hydrograph and followed similar temporal dynamics to that of caffeine and fecal indicator bacteria, which are chemical and biological markers of potential fecal pollution. Results from this study demonstrate that tracking of TRP fluorescence intensity and TRP:humic ratios, with turbidity correction of sensor outputs, may be an appropriate warning tool for rapid monitoring of sewage or other bacterial inputs to aquatic environments.