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Investigative research

 

Groundwater modelling

 

The Downs & Harbours (D&H) programme financially contributed to the South Downs Collaborative Modelling project (Amec Foster Wheeler), completed in 2014, in exchange for the inclusion of three of Portsmouth Water’s (PW) boreholes. The model was principally concerned with nitrate, the leading concern of both D&H and PW. Subsequently, PW commissioned Amec Foster Wheeler to extend the model to include its remaining sources.

 

Although the D&H programme was the principal financial contributor to this modelling (PW’s supply area broadly sits within its operational area), it relied on PW’s involvement to assist Amec FW to produce the final model.

 

The outcomes of the model include:

 

  • The establishment of the total catchment areas of groundwater sources supplying abstraction boreholes;

  • Total catchments of groundwater supplying spring sources;

  • Total catchment supplying the River Itchen abstraction;

  • Derivation of groundwater catchment zones;

  • Surface water catchments to groundwater sources (springs and boreholes);

  • Nitrate trend models for groundwater sources; and

  • Source Apportionment Modelling for groundwater sources, showing the percentage of nitrate derived from different activities – e.g. types of agriculture, landfill and forestry.

 

Particular emphasis was placed on the visual accessibility of many of the modelled outputs to provide relatively easy identification of areas of concern. This would enable more accurate targeting measures to be developed to combat diffuse nitrate losses to both surface drainage (rivers) and to groundwater.

 

Below: an example of an output shows the three delineated catchment zones (Inner, Outer and Total Capture) that supply Havant Springs. It also shows the fraction of water derived from each square km in the zones that contributes to the Springs, the source of drinking water for Havant and Portsmouth.

 
 

 

Surface water risk mapping

 

Although connectivity between swallow holes and PW’s boreholes has been seen in the past, very little is known about the potential risks that the significant number of karst features represent to drinking water, rivers and local harbours. Risks not only include nitrate but other contaminants, including faecal pathogens (e.g. cryptosporidium), pesticides and domestic heating oil.

 

As a result, the D&H partnership began working with Rezatec in 2015 to deliver a bespoke GIS-based data ‘portal’ (web page) to determine what level of risk may be present. A ‘Proof of Concept’ (PoC) model was jointly developed by D&H, other PW staff and Rezatec to test the model.

 

Its principal aims were to:

 

  • Obtain a better understanding of what the risks were – mainly nitrate related to agriculture and manure heaps – and where they could be found;

  • See how such risks were hydrologically linked to swallow holes and other karst features (such as dolines) that, in turn, could be hydrologically linked to PW pumping station boreholes and affecting groundwater in general;

  • Create visually-based and easily understandable risk maps and spreadsheets to present ‘evidence’ to stakeholders and therefore help bring about improvements in land management;

  • To test the efficacy of Rezatec’s approach.

 

The PoC has now been developed further to analyse all 20 SPZs within Portsmouth Water’s supply area, with the new ‘portal’ displaying an interactive risk map. Overlaying features can be independently or jointly selected, showing the relationship between for example:

 

  • A base layer map

  • Satellite imagery (e.g. Google or Bing type maps)

  • Geology and soil type

  • Principal agriculture type(s)

  • Manure heaps

  • Borehole locations

  • Karst features or chalk pit locations and their associated catchment areas

  • Surface flow pathways

  • Surface flow accumulation areas

  • Rainfall/run-off model

 

An example output from the portal is shown below displaying some of the above features within a section of PW’s area of supply.

 

The groundwater model and surface-based risk mapping undertaken will enable a greater insight into the key ‘at risk’ areas within both the D&H’s project and PW’s supply areas. This will enable more targeted application of mitigation practices and assist the monitoring of outcomes.

Further work

The D&H partnership is highly aware of the importance of continuing any investigative research where it provides greater understanding of - and the means of tackling - potential risks. Further development to field-level assessments and modelling is projected in a number of forthcoming projects.

Further research is increasingly likely to be funded by PW’s Catchment Management programme, although D&H will continue to financially support research where appropriate. Its principal remit will be to continue to be the main deliverer of on-the-ground services and advice.

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