Epilithon isotope composition as an environmental archive in rivers receiving wastewater

Epilithon is a complex community of autotrophic and heterotrophic organisms that includes inert, organic and inorganic material and is attached to the surface of submersed rocks. Water samples collected in the Grand River (southwestern Ontario) in April 2011 showed that ammonium concentrations decreased downstream, whereas nitrate varied, largely dependent on weather conditions (concentrations of both chemical species were higher during winter). Epilithon δ15N-TN downstream from the Kitchener wastewater treatment plant oscillated between 0.4 to 23.2‰, and δ13C-TC around -27‰. The wastewater treatment plant effluent consisted of δ15N-NO3- between 12 and 16‰, with a decreasing trend as it traveled downstream; δ15N-NH4+ became enriched downstream (as high as 31‰). Average values for δ13C-DIC were -10.1‰ and δ13C-DOC -26.8‰. It is proposed that the nitrogen and carbon isotope composition of epilithon could be used as a short- or medium-term environmental archive, as it reflects in-stream processes, such as ammonia oxidation, in a river impacted by treated wastewater. The interpretation provided here was limited due to the ample range of events and potential sources, specifically when the nitrogen isotopic composition of nitrate and ammonium was similar. Epilithon is easily collected, processed and analysed and proved to be valuable tool to describe changes in river and stream geochemistry.

The influence of concrete on the geochemical qualities of urban streams

The geochemical signature of freshwater streams can be used to determine the extent and nature of modification to stream water geochemistry due to urban development. This approach used the Gibbs (1970) diagram as a model for evaluation of changes to ionic composition linked to urban development. In this multi-year study, the geochemistry of 21 waterways in the Georges River catchment, Sydney, were monitored and compared with the level of urban development as measured by sub-catchment imperviousness and directly connected imperviousness. The results reflect a strong relationship between the intensity of sub-catchment urban development and stream geochemistry. All major geochemical attributes increased with escalating levels of urban development. The largest increase was for bicarbonate, which increased 18 times from a mean of 6.4 mg L–1 at non-urban streams to a mean of 118 mg L–1 at urban streams. Similarly, mean concentrations of calcium increased by 14 times (from 2 to 27.9 mg L–1). Mean salinity was enriched in the most urban streams, compared with non-urban streams, by more than 6 times. We attribute this, in part, to the influence of urban geology, notably concrete stormwater infrastructure. Changes in stream geochemistry due to urban development are an important element of the urban stream syndrome.

Geochemistry of streams from Byers Peninsula, Livingston Island

In January and February 2009, a series of water samples were collected from streams on Byers Peninsula. These samples were analysed for major elements and δ18O to determine the role of lithology and landscape position on stream geochemistry, and to understand better the hydrology (i.e. residence time of water) of these systems. Precipitation chemistry is enriched in Na+, as are the streams located close to the coast. Streams originating from inland locations have much higher percentages of Ca2+. In contrast, Mg2+ varied little, though streams that are in greater contact with volcanic-derived soils have slightly higher concentrations. Anion percentages varied greatly between streams with SO42- ranging from 5% to 45% of the anion composition. Dissolved Si concentrations as high as 141 μM were observed. All these data suggest that active chemical weathering is occurring in this region. A time series over 13 days at one stream showed little variation in major element geochemistry. The δ18O of precipitation samples collected over this same period varied by ∼10‰ while the majority of stream samples varied less than ∼1.5‰. These data indicate that the stream waters represent mixtures of precipitation events, melting snow and water from the subsurface that had gained solutes through chemical weathering.

A new type of water pollution: concrete drainage infrastructure and geochemical contamination of urban waters

Stormwater and other urban runoff is often conveyed by concrete infrastructure and it is plausible that the chemistry of urban streams is modified by the leaching of minerals from this infrastructure. We tested this hypothesis by analysing major anions, cations and other chemical variables from urban and reference freshwater streams in northern Sydney. Urban streams tended towards neutral pH whereas non-urban reference streams were acidic. Bicarbonate levels were more than 10 times higher and calcium concentrations were more than six times higher in urban streams than reference streams. Experimental analysis revealed that the chemistry of rainwater changed when passed through concrete pipes and down concrete gutters, suggesting dissolution of cement products from various concrete materials used for urban drainage. This study concluded that the use of concrete – particularly its application for urban drainage – is responsible for some of the modifications to urban stream geochemistry. Thus, urban geology should be considered as an important factor that contributes to the urban stream syndrome.

Tropical glacier meltwater contribution to stream discharge: a case study in the Cordillera Blanca, Peru

Discharge measurements, climate observations and hydrochemical samples gathered monthly (1998/99) in the Yanamarey and Uruashraju glacier-fed catchments of the Cordillera Blanca, Peru, permit an analysis of the glacier meltwater contribution to stream-flow. These glacier catchments feed the Río Santa, which discharges into the Pacific Ocean. Based on a water-balance computation, glacier melt contributes an estimated 35% of the average discharge from the catchments. For comparison, a volumetric end-member mixing model of oxygen isotopes shows glacier melt contributes 30–45% to the total annual discharge. Based on stream geochemistry, discharge from the Yanamarey glacier catchment provides 30% of the annual volume discharged from the Querococha watershed, which is <10% glacierized. By analogy, the larger Río Santa watershed, also <10% glacierized, receives at least 12% of its annual discharge from melting glacier ice. Tributary watersheds to the Río Santa with larger fractions of glacier cover have less variable runoff and enhanced discharge, demonstrating that the glaciers effectively buffer stream discharge seasonally. With continued glacier melting, stream-flow will likely become more variable, and there will be less dry-season runoff.