Abstract
Groundwater pollution by agrochemicals such as nitrogen fertilizers can cause complex biogeochemical transformations to take place in groundwater-dependent ecosystems. To explore the interaction between nitrogen load and groundwater-dependent, spring-fed ecosystems, a study was conducted in Latvia in an area of suspected high nitrate (NO3−) vulnerability due to its geological settings. A map of NO3− vulnerability along the margins of the carbonate aquifer in Latvia is presented. The map is based on a conceptual model that was developed during an extensive case study involving hydrological, hydrochemical, and habitat investigation of springs discharging from a karst aquifer and spring-fed ecosystems. Areas that should be prime targets for restricting fertilizer application are highlighted on the map. Although the case study revealed increased nitrogen pollution (up to 51 mg L−1, standard deviation of 9 mg L−1, in the springs discharging from the karst aquifer), no clear evidence of adverse effects due to NO3− pollution on the groundwater-dependent ecosystems using biotic indicators was found, highlighting the resilience of spring-fed ecosystems against high nitrogen inputs. In the case study, downstream groundwater-dependent ecosystems retained 70% of the reactive nitrogen during the vegetation season, but only a small proportion during the cold season. Thus, NO3− pollution can be partly mitigated by restoring wetlands along valley slopes where natural groundwater discharge takes place. The conceptual model developed for groundwater NO3− vulnerability is applicable to other areas in the Baltic region and other places with similar climatic and geological conditions.
Highlights
Groundwater pollution by agrochemicals such as nitrogen (N) fertilizers (Böhlke 2002) is a global problem that is causing the deterioration of the quality of water resources and is degrading groundwater-dependent ecosystems and other wetlands (Wulff et al 2007; Gruber and Galloway 2008; Rockström et al 2009; Giakoumakis et al 2013)
The map is based on a conceptual model that was developed during an extensive case study that involved hydrological, hydrochemical and habitat investigation of springs discharging from a karst aquifer and connected groundwater-dependent ecosystems
The habitat types were determined according to the vegetation uniformity and abiotic conditions, and their relevance to the Latvian national interpretation (Auniņš et al 2013) of the habitats listed in Annex I of European Union (EU) Habitats Directive 92/43/EEC (Annex I habitat types) in 125 sample plots that were 50 × 50 cm large
Summary
Groundwater pollution by agrochemicals such as nitrogen (N) fertilizers (Böhlke 2002) is a global problem that is causing the deterioration of the quality of water resources and is degrading groundwater-dependent ecosystems and other wetlands (Wulff et al 2007; Gruber and Galloway 2008; Rockström et al 2009; Giakoumakis et al 2013). The main pathway of N attenuation in groundwater is bacterial reduction–denitrification (Rivett et al 2008) to nitrite (NO2−), nitric oxide (NO), nitrous oxide (N2O) and, molecular N 2 This process can take place in the absence of molecular oxygen, wherein electron donors are organic carbon or sulfide minerals (Böhlke 2002; Hansen et al 2014). Excess nutrient inputs into natural ecosystems can lead to eutrophication and loss of biodiversity, but it can be triggered by excess SO4−2 inputs as well (Smolders et al 2006; Geurts et al 2009; Cirkel et al 2014)
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