Better water quality while saving costs and carbon? Water quality models SAGIS and SIMCAT are the answer

If something good has come out of the COVID-19 pandemic and all the lockdowns we have been through, it is that it has made the public more aware of the value of their environment. Coupled with the interest of the government to deliver a ‘Green Brexit’ and a green industrial revolution, it’s clear that there is huge pressure to improve the status of the world we live in, including the water in our rivers.

Climate change, population growth and other emerging issues, like chemicals and sewage in the environment, make traditional approaches to environmental management obsolete and new ideas and methods are now required to achieve such an ambitious goal. Read on to find out how…

Historically, water quality regulation has been completed on a site-by-site basis in which each location (such as a sewage treatment works discharge) was assessed and permitted individually. This has resulted in a huge effort from all parties involved with little room for efficiency savings or for embedding new and better practices.

In the last decade, the Environment Agency (EA) and other regulators such as Natural England have encouraged water companies and other parties involved in water quality planning to think beyond the old ways of working and to bring innovative ideas to the table. One of the most successful innovative strategies is the ‘catchment-based approach’.

A catchment is an area of land through which rain drains into a river, lake or reservoir, or even down into the soil and into groundwater. It could be very large, such as a major river ending at an estuary, or it could be much smaller like a sub-catchment of a river.

The ‘catchment-based approach‘ is an initiative that brings together government agencies, local authorities, water companies and businesses. They identify issues, agree what should be done, put them into practice to maximise the natural value of our environment.

It highlights the importance of the management of land and surface water interaction as a system, putting the emphasis on actions that reflect how nature actually works.

This approach is one of the pillars of the government’s 25-year Environment Plan, because it increases resource efficiency, and reduces pollution and waste, among other things. This is now even more significant as the government announced its ten-point plan for a green industrial revolution aiming to “Building back better, supporting green jobs, and accelerating our path to net-zero”.

Population growth is a significant challenge for the UK, with an increase of over 10 million people in the last 20 years and another 5 million more expected in the next 20 years. Much of this growth will be happening close to areas that are already under a lot of pressure, such as rivers, many of which are failing to meet the standards set by the Water Framework Directive.

Another threat to water quality is climate change. Forecasting the probability and degree of climate change impacts on water quality is problematic because of the broad range of natural variability in hydrology, chemistry and ecology.

Overall, we are expecting wetter winters and drier summers. The anticipated lower minimum flows result in lower dilution and hence higher concentrations of pollutants such as sewage downstream of discharges. The EA has estimated that concentrations of phosphorus will reduce during the winter months but increase during summer and autumn.

The opposite pattern is estimated for nitrate, with increased levels in the winter, decreasing in the summer months. All of this will promote algal blooms in rivers and reservoirs, which could affect water quality (e.g. oxygen, pH) with negative effects on aquatic fauna.

With so many variables and possibilities to consider, water quality modelling at a large catchment scale is a valuable tool to manage the wide-ranging water quality issues.

SIMCAT (SIMulation of CATchments) is the EA’s water quality river model. It is used to establish and assess discharge permits at a catchment scale and to calculate the source apportionment (i.e. relative importance) of both point source inputs (such as sewage treatment works, which enter a river at a single site via a pipe) and diffuse pollutant loads (like runoff from urban areas, agricultural land or septic tanks, which may enter a river at various different points).

With a modelling coverage of the whole of England, Wales and Scotland, one of the advantages of this model is the possibility of quickly creating different scenarios (such as a range of discharge flows or quality) which can be compared to other scenarios for determining the impact and what mitigation measures are needed.

Because of the catchment scale at which this model operates, it makes it very easy to simultaneously see the effect of several sources of pollution, providing vital information for the ‘catchment-based approach’.

In the example illustrated above, the graph on the left shows the volume of water flowing along a stretch of a river from the headwaters to the estuary. The graph on the right indicates the concentration of BOD (biochemical oxygen demand) along the same river. Step changes in the river flow can be due to discharges and the confluence with a tributary (if it goes up) or an abstraction (if it goes down). There are some interesting points on this SIMCAT graph:

Arrow 1. Flow increase at this point coincides with a 60% increase in BOD concentration. This inflow is a tributary, which highlights that the tributary is a major source of pollution and is therefore where mitigation needs to be concentrated.

Arrow 2. Flow increase reflects a discharge improving the river quality as it reduces the concentration of BOD. This is a fairly “clean” discharge, so mitigation isn’t required.

SAGIS (Source Apportionment Geographical Information System) uses the outputs from SIMCAT to provide a broader summary of inputs from the different contributing sectors. This allows water companies and regulators to have a shared system to create effective, evidence-based mitigation measures. These are used by asset managers and catchment planning teams to develop mitigations that follow the “polluter pays” principle.

Each contributor to a pollutant can see its contribution to the total, and therefore its responsibility for applying mitigation. This ensures a fair distribution of the obligations to improve water quality across all responsible sectors. SAGIS uses the ArcGIS platform to visually represent the complex results in a user-friendly manner.

Other tools in SAGIS include a decision support tool that can evaluate the efficiency of measures to manage inputs from both point and diffuse sources, or combinations of both, to support catchment-based decision making.