Nitrate Trending – Chalk aquifers

Case Study

ESI, now part of Stantec, was commissioned by several water companies to review trends of increasing nitrate concentrations in a large number of boreholes. This includes sources abstracting from the Chalk aquifer.

Background

Concentrations of nitrate in groundwater have increased significantly over the last few decades, largely in response to increased application of nitrate to land. Initially water companies could avoid additional treatment costs by blending this groundwater with water from lower nitrate sources. However, as trends have continued to rise, options for blending have reduced. A particular issue for sources abstracting from the Chalk aquifer is that nitrate concentrations respond to antecedent recharge/groundwater levels and are thus strongly seasonal.

 

Meanwhile, the Water Framework Directive puts an onus on water companies and regulators to work together to solve water quality issues at source, rather than continuing to rely on ‘end of pipe’ solutions. Both of these factors require an improved understanding of current and future predicted trends in water quality. Understanding the variability in nitrate concentration is important as compliance is needed throughout the year.

Background

ESI was contracted by Thames Water in 2015 to review and simulate trends in nitrate concentration at 23 chalk groundwater sources. Due to the seasonal variation in nitrate that is observed at chalk boreholes, more complicated modelling techniques are required than for the sandstones. We updated and improved our nitrate trend model to incorporate these seasonal variations (as partof an internal R&D project) and used this to deliver the nitrate element of thework.

 

Through the findings of the initial assessment, we were able to adapt outputs to suit the client’s needs, which included: a review of the distribution and contribution of nitrate to each abstraction through spatial modelling; the ‘contribution to abstraction yield’, ‘travel time’, ‘change in nitrate loading per year’ and ‘contribution to N in the abstraction’.

Nitrate Trending – Chalk aquifers_1

As with many similar ‘investigative’ projects, ESI were able to be flexible and adaptive to the requirements of the client, which evolved as we learned more about the catchments during the project. ESI was also able to produce over 15 model outputs (including long-term and seasonal forecasted trends), revised catchment areas, spatial analysis outputs, a headroom assessment (to support catchment management planning), training and a manual in just over 3 months.

The Outcome

The results of these investigations provided Thames Water with both the evidence and a method for prioritising its further investigations and proposed management at these sources. It also gave the Environment Agency confidence that these measures were being applied appropriately.

Nitrate Trending – Chalk aquifers

ESI, now part of Stantec, has carried out similar nitrate trending work for four other water companies and our trending tool has now been used on over 100 sources.