Controls on species distribution and biogeochemical cycling in nitrate-contaminated groundwater and surface water, southeastern Australia

S Michael Adelana; Michael W Heaven; P Evan Dresel; Khageswor Giri; Mark Holmberg; George Croatto; John Webb

doi:10.1016/j.scitotenv.2020.138426

Controls on species distribution and biogeochemical cycling in nitrate-contaminated groundwater and surface water, southeastern Australia

Sci Total Environ. 2020 Jul 15:726:138426. doi: 10.1016/j.scitotenv.2020.138426. Epub 2020 Apr 8.

Authors

S Michael Adelana¹, Michael W Heaven², P Evan Dresel³, Khageswor Giri⁴, Mark Holmberg³, George Croatto⁵, John Webb⁶

Affiliations

¹ Agriculture Victoria Research, Department of Jobs, Precincts and Regions, 5 Ring Road, Bundoora, Victoria 3083, Australia. Electronic address: Michael.Adelana@agriculture.vic.gov.au.
² Agriculture Victoria Research, Department of Jobs, Precincts and Regions, 1301 Hazeldean Road, Ellinbank, Victoria 3821, Australia.
³ Agriculture Victoria Research, Department of Jobs, Precincts and Regions, Cnr Taylor Street & Midland Highway, Bendigo, Victoria 3554, Australia.
⁴ Agriculture Victoria Research, Department of Jobs, Precincts and Regions, 5 Ring Road, Bundoora, Victoria 3083, Australia.
⁵ Agriculture Victoria Research, Department of Jobs, Precincts and Regions, Terrace 4 Ernest Jones Drive, Macleod, Victoria 3085, Australia.
⁶ Department of Ecology, Environment and Evolution, La Trobe University, Bundoora, Victoria 3086, Australia.

PMID: 32320873
DOI: 10.1016/j.scitotenv.2020.138426

Abstract

A detailed study of groundwater and surface water nitrate over four seasons across an area of varied landuse provided insights into the mechanisms that underlie accumulation and transport of nitrate. High nitrate concentrations found in a significant percentage of surface water and shallow groundwater samples are due to anthropogenic contamination. Statistics (PCA, ANOVA, parsimonious model and general linear regression) were used to explore the relationship between NO₃^- and land use, and confirmed that areas of high NO₃^- concentration are associated with dairy pasture and horticulture. Seasonally, NO₃^- levels are greater during winter, the wettest part of the year. Values of δ¹⁵N showed that most nitrate is sourced from livestock waste, with a smaller contribution from synthetic fertilizer. Direct wash-off of animal waste from dairy farms results in higher NO₃^- concentrations in surface water than in groundwater. Denitrification is an important NO₃^- attenuation mechanism which reduces NO₃^- to NH₄, as demonstrated by the PCA analysis, which showed positive correlation of NO₃^- concentrations with dissolved oxygen and negative correlations with NH₄⁺, Fe²⁺and Mn²⁺; the latter two species may act as the electron donors necessary for reduction of NO₃^-. The often high NO₃^- concentrations in shallow groundwater are decreased by denitrification, which can occur at relatively shallow depths (<3 m). The relatively small NO₃^- concentrations in deeper groundwater are due partly to denitrification, but more to originally lower NO₃^- concentrations, as the age of deeper groundwater shows that it was recharged before agriculture was established in the study area. Overall, the study demonstrates the usefulness of hydrogeochemical characterisation and multivariate statistics in the evaluation of impacts of agricultural land-use on regional N cycling. In particular, the results show that efforts to mitigate NO₃^- pollution from farms should concentrate more on wash-off of animal waste than the contribution of nitrogenous synthetic fertilizer.

Keywords: Agricultural land-use; Biogeochemical reactions; Denitrification; Elevated nitrate; Regression analysis.