Nitrate fluctuations at the water table: implications for recharge processes and solute transport in the Chalk aquifer

Abstract

This study investigates fluctuations in nitrate concentration at the water table to improve understanding of unsaturated zone processes in the Chalk aquifer. Sampling was conducted using a novel multi-level sampler during periods of water table rise over five years. The sampler allowed near continuous sampling at a vertical resolution of 0.05 m. Nitrate concentration increased as the water table seasonally recovered, with similar interannual trends with depth. The rising water table activated horizontal fractures facilitating the delivery of water elevated by up to 10 mg/L of nitrate with respect to the adjacent groundwater below. These fractures are considered to activate via piston displacement of water from the adjoining matrix. Hydrograph analysis identified sixteen events which significantly perturbed the water table within 24-48 hours of rainfall. Consistent nitrate concentrations indicate recharge through persistent fracture flow from the surface was not generally the primary driver of the rapid water table response during these events. Instead, the response was attributed to the piston displacement of porewater immediately above the water table. However, a single event in November 2012 delivered relatively dilute recharge indicating rapid persistent fracture flow following rainfall was possible to a depth of 14-15 m. Decreases in porewater nitrate concentration around fracture horizons and the dilution of many groundwater samples with respect to porewaters indicate a fresher source of water at depth. This was considered most likely to be a result of near surface water bypassing the matrix frequently because of widespread mineralisation on fracture surfaces, which retard water and solute exchange. Therefore, persistent fracture flow maybe considered a frequent process in, operating independently of the matrix, and is not necessarily event driven. Keywords: nitrate, chalk, recharge, piston displacement, fracture flow, contaminationThis article is protected by copyright. All rights reserved. IntroductionUnderstanding the mechanisms of contaminant migration through the unsaturated zone is important for assessing the degree of attenuation and risk to aquifers. This is particularly relevant for fractured aquifers which can sometimes exhibit rapid delivery of contaminants from the surface to the water table (Nimmo et al. 2002, Mellander et al. 2013. The Chalk aquifer is the most important source of freshwater in Northwestern Europe and fracture flow can occur within the unsaturated zone (Downing et al. 1993). However, there is ongoing uncertainty over its significance (Price et al. 2000, Ireson et al. 2006, Van den Daele et al. 2007, Ireson and Butler 2011, Keim et al. 2012) and persistence with depth (Gooddy et al. 2001, Haria et al. 2003, Ireson et al. 2012).The Chalk is a fine-grained soft limestone with low permeability, moderate to high porosity and very small pore diameters. It is commonly intersected by high angle joints and numerous horizontal discontinuities associated with bedding and seconda...

Intra‐event concentration–discharge relationships affected by hydrological connectivity in a karst catchment

“…For soil‐enriched solutes, diffuse percolation of soil water to small fractures in depression increases the availability of solutes in small fracture water (Sorensen et al, 2015; Yue et al, 2019). In addition, higher water level in conduits at the beginning of a flood can lead water flux with high concentrations (showing in Figure 7) from conduits to small fractures, known as the ‘bidirectional exchange’ between conduit and small fracture in karst critical zone (Chen et al, 2018; Zhang et al, 2017).…”

Transfer of water and contaminants in the Chalk unsaturated zone – underground quarry of Saint-Martin-le-Nœud

“…For NO − 3 , El Gaouzi et al (2013) observed concentrations between 34 and 49 mg l −1 in boreholes or springs in the Paris Basin. Sorensen et al (2015) found nitrate concentrations ranging from 30 to 60 mg l −1 for the Chalk aquifer in the UK. For pesticides, Baran et al (2008) At Saint-Martin-le-Noeud, the concentrations of nitrate, atrazine and DEA vary spatially along the quarry both in lakes and percolating water.…”

A Critical Review of Groundwater Table Fluctuation: Formation, Effects on Multifields, and Contaminant Behaviors in a Soil and Aquifer System

“…129 Moreover, the concentrations of NO 3 − increased with an uplifted groundwater table in the U.K. over 5 years, and researchers implied that varied NO 3 − concentrations were also determined by fracture displacement and the amplitude and frequency of GTF. 130 The variation of groundwater physicochemical properties, such as DO, pH, and E h , caused by GTF significantly influences the migration and transformation of nitrogenous compounds (NO 3 − , NH 4 + , and NO 2 − ) in unsaturated soil and saturated aquifers. In a case study in the Luoyang area, China, the nitrate nitrogen concentration increased with a decreasing groundwater table, whereas the ammonium nitrogen concentration decreased because of the variations in the physicochemical properties of the soil and groundwater.…”