Nitrate vulnerability assessment of aquifers

Abstract

Protection of groundwater against nitrate has a high priority in a country like Denmark with intensive agricultural production and with drinking water production based on groundwater. This paper presents a Site-specific Concept for Aquifer Nitrate Vulnerability Assessment (SCANVA) aimed at protecting groundwater resources. SCANVA is a qualitative dynamic concept adjusted to the specific study area depending on the hydrogeological and geochemical conditions. It comprises a synthesis of very detailed site-specific geological, geophysical, hydrogeological and chemical data, and a definition of nitrate vulnerability founded on redox conditions in the ground. Data are used to interpret the geological architecture of the subsurface, the groundwater flow and the geochemical groundwater conditions. These interpretations are integrated to assess nitrate vulnerability of the aquifers in three dimensions and identifying nitrate-vulnerable zones. The paper demonstrates the practical use of the concept in a Danish intensive agricultural area with a glacially formed landscape. Glaciotectonic has led to a high geological heterogeneity and very complex composition of thrusted and folded layers, which to a large extent affect the nitrate vulnerability of the aquifers. SCANVA can be directly applied to specific hydrogeological conditions anywhere with intensive N loss from agriculture, a groundwaterbased drinking water supply, nitrate reduction in the ground and glacially dominated landscapes and deposits. Potentially, the concept might also be adapted to other specific substances, dominating geochemical processes, and geological settings.

Denitrification in Shallow Groundwater Below Different Arable Land Systems in a High Nitrogen‐Loading Region

“…Unfortunately, the relationship between denitrification and the particulate organic C concentration was not investigated in this study. Inorganic compounds, such as pyrite (FeS 2 ) and Fe(II) silicates, may also be used by denitrifying organisms and have been found to play an important role in groundwater denitrification in some reports (Böhlke et al, 2002;Hansen et al, 2016;); however, it could not be a major contribution in our study site because of the low release of ferrous and sulfate from the experimental soil (Jiang et al, 2012).…”

Groundwater nitrate response to sustainable nitrogen management

“…extensive use of catch crops, or restrictions on the N application level to specific crops. However, in order to obtain a sufficient groundwater protection level with the new Danish N regulation there is a need for development of new hydro-geochemical mapping tools 21 , 38 in order to be able to identify the most N sensitive agricultural fields at a very fine spatial scale (hectares-scales). Introduction of locally targeted N measures is also challenged by other factors such as: prices of land, farmers’ resistance, challenges related to reallocation of land use between farms, collaboration of farmers, citizens and authorities, as well as the uncertainty on the local hydro-geochemical vulnerability assessment of the fields.…”

“…Therefore, nitrate present in groundwater in rural areas is strongly linked to the amount of N applied to agricultural land, and to the N surplus in particular 18 . The resulting nitrate concentration in groundwater depends on 1) net precipitation and nitrate leaching from land use, 2) redox conditions and nitrate reduction for example with pyrite oxidation in the groundwater system 19 , 20 , and 3) hydrogeological conditions and residence time in the groundwater system 21 . Long-term effects of N losses from agricultural land on groundwater nitrate content have been reported from all over the globe 22 – 26 .…”

3D multiple point geostatistical simulation of joint subsurface redox and geological architectures

“…In a simple case with only vertical infiltration, nitrate concentrations in aquifers decrease with an increasing depth along three sequential redox zones (Kim et al, 2019;Wilson et al, 2018): 45 1) Oxic zone: Nitrate conditions are equal to the leaching from the soil because of the oxic conditions preventing reduction 2) Nitrate reducing zone: Nitrate decrease with increasing depth due to ongoing reduction of nitrate 3) Reduced zone: Nitrate free zone due to complete reduced redox conditions The redox conditions of the subsurface has been widely investigated using various approaches focusing on different redox 50 sensitive chemical compounds such as nitrate, iron, sulphate, pyrite, organic matter, arsenic, uranium, and some organic contaminants: 1) process-based approaches (e.g. (Abbaspour et al (2007); Hansen et al (2014aHansen et al ( ,2016a; Lee et al, (2008)), 2) geostatistical methods (e.g., Kriging;Ernstsen et al (2008); Goovaerts et al (2005) ;Lin, (2008)) and 3) machine learning (Close et al, 2016;Koch et al, 2019;Nolan et al, 2015;Ransom et al, 2017;Rosecrans et al, 2017;Tesoriero et al, 2015; challenge for 3D modeling and interpretations between geological point data may lead to large uncertainties (Wellmann and Caumon, 2018).…”

“…tTEM is, therefore, ideal for high-resolution mapping when focusing on the uppermost 50 to 70 m of the subsurface. None of the previous studies has investigated the geological and redox architecture simultaneously although these two are related and sometimes coevolved (Grenthe et al, 1992;Hansen et al, 2016a;Wilkin et al, 1996;Yan et al, 2016).…”