Steven R. Silva

Pathways for nitrate release from an alpine watershed: Determination using δ15N and δ18O

d 18 O(NO3) indicated that about half of the nitrate in stream water was the product of microbial nitrification; at other times that amount was greater than half. Springs emerging from talus deposits had high nitrate concentrations and a seasonal pattern in d 18 O(NO3) that was similar to the pattern in the streams, indicating that shallow groundwater in talus deposits is a likely source of stream water nitrate. Only a few samples of surface water and groundwater collected during early snowmelt and large summer rain events had isotopic compositions that indicated most of the nitrate came directly from atmospheric deposition with no biological assimilation and release. This study demonstrates the value of the nitrate double-isotope technique for determining nitrogencycling processes and sources of nitrate in small, undisturbed watersheds that are enriched with inorganic nitrogen. INDEX TERMS: 1806 Hydrology: Chemistry of fresh water; 1871 Hydrology: Surface water quality; 1803 Hydrology: Anthropogenic effects; 1854 Hydrology: Precipitation (3354); KEYWORDS: nitrogen, oxygen, isotopes, alpine, watershed, deposition

Nitrogen losses from dairy manure estimated through nitrogen mass balance and chemical markers

Ammonia is an important air and water pollutant, but the spatial variation in its concentrations presents technical difficulties in accurate determination of ammonia emissions from animal feeding operations. The objectives of this study were to investigate the relationship between ammonia volatilization and delta15N of dairy manure and the feasibility of estimating ammonia losses from a dairy facility using chemical markers. In Exp. 1, the N/P ratio in manure decreased by 30% in 14 d as cumulative ammonia losses increased exponentially. Delta 15N of manure increased throughout the course of the experiment and delta15N of emitted ammonia increased (p<0.001) quadratically from -31 per thousand to -15 per thousand. The relationship between cumulative ammonia losses and delta15N of manure was highly significant (p<0.001; r2=0.76). In Exp. 2, using a mass balance approach, approximately half of the N excreted by dairy cows (Bos taurus) could not be accounted for in 24 h. Using N/P and N/K ratios in fresh and 24-h manure, an estimated 0.55 and 0.34 (respectively) of the N excreted with feces and urine could not be accounted for. This study demonstrated that chemical markers (P, K) can be successfully used to estimate ammonia losses from cattle manure. The relationship between manure delta15N and cumulative ammonia loss may also be useful for estimating ammonia losses. Although promising, the latter approach needs to be further studied and verified in various experimental conditions and in the field.

Carbon isotopic constraints on the contribution of plant material to the natural precursors of trihalomethanes

The d 13 C values of individual trihalomethanes (THM) formed on reaction of chlorine with dissolved organic carbon (DOC) leached from maize (corn; Zea maize L.) and Scirpus acutus (an aquatic bulrush), and with DOC extracted from agricultural drainage waters were determined using purge and trap introduction into a gas chromatograph-combustion-isotope ratio monitoring mass spectrometer. We observed a 16.8- diAerence between the d 13 C values of THM produced from the maize and Scirpus leachates, similar to the isotopic diAerence between the whole plant materials. Both maize and Scirpus formed THM 12- lower in 13 C than whole plant material. We suggest that the low value of the THM relative to the whole plant material is evidence of distinct pools of THMforming DOC, representing diAerent biochemical types or chemical structures, and possessing diAerent environmental reactivity. Humic extracts of waters draining an agricultural field containing Scirpus peat soils and planted with maize formed THM with isotopic values intermediate between those of maize and Scirpus leachates, indicating maize may contribute significantly to the THM-forming DOC. The diAerence between the d 13 C values of the whole isolate and that of the THM it yielded was 3.9-, however, suggesting diagenesis plays a role in determining the d 13 C value of THM-forming DOC in the drainage waters, and precluding the direct use of isotopic mixing models to quantitatively attribute sources. # 1999 Elsevier Science Ltd. All rights reserved.

Water chemistry of Shoal Creek and Waller Creek, Austin, Texas, and potential sources of nitrate

The report gives an overview of the water quality and mineral content of both Shoal Creek and Waller Creek.

Characterization of the Microcystis Bloom and Its Nitrogen Supply in San Francisco Estuary Using Stable Isotopes

A suite of particulate and dissolved organic and inorganic stable isotopes were needed to determine the source of the nutrients and cells that initiate and sustain the toxic cyanobacteria bloom of Microcystis in San Francisco Estuary. Particulate and dissolved inorganic and organic matter in water and plankton samples were collected biweekly during Microcystis blooms in 2007 and 2008. Stable isotopes for particulate and dissolved organic matter, nitrate, and water (POM-δ13C, POM-δ15N, DOC-δ13C, C/N ratio, NO3-δ15N, NO3-δ18O, H2O-δ18O and H2O-δ2H) were compared with Microcystis cell abundance, dissolved organic carbon, chlorophyll a, and toxic total microcystins concentration, as well as physical and chemical water quality variables, including streamflow. The isotopic composition of particulate organic matter, nitrate, and water differed for the Sacramento and San Joaquin Rivers and varied along the salinity gradient. The variation of particulate organic matter and water isotopes suggested Microcystis primarily entered the estuary from the San Joaquin and Old Rivers, where it was most abundant. Nitrate isotopes along with streamflow variables indicated that the San Joaquin River was a source of nitrate to the estuary. However, stable isotope comparison of the nitrogen in Microcystis cells with the dissolved inorganic nitrate in the San Joaquin River indicated that nitrate was not the primary source of nitrogen that supported the bloom. Instead, ammonium from the Sacramento River was the likely sole source of the nitrogen for most of the bloom. Selective uptake of ammonium may have further contributed to the magnitude of the Microcystis bloom which increased with the percent of ammonium within the total dissolved inorganic nitrogen pool.

Applications of Stable Isotopes for Regional to National-Scale Water Quality and Environmental Monitoring Programs

Isotopes are a potentially powerful component of monitoring and assessment programs aimed at quantifying and mitigating alterations to environments from human activities. In particular, isotopic techniques have proved useful for tracing sources and sinks of various pollutants in large river basins, wetlands, and airsheds. Many of these studies have been conducted at the regional to national scale by building on existing large-scale water, air, and ecological monitoring programs managed by federal and state agencies, and demonstrate the usefulness of isotopes as a complement to standard chemical and hydrological mass balance methods. This chapter presents an overview of how nitrate, particulate organic matter, and water isotopes can be used to interpret spatial patterns and temporal changes in pollution sources, biogeochemical processes, and ecosystem function in watersheds, at the regional to national scale. Examples from several recent and ongoing studies are presented. From the insights developed using varied sampling strategies and isoscapes, we suggest guidelines for future studies in biologically active and human-impacted rivers.