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Why less polluted air is leading to the browning of rivers

About the blog

Don Monteith
Environmental Change Network and UK Upland Waters Monitoring Network coordinator, UK Centre for Ecology & Hydrology.
  • Why less polluted air is leading to the browning of rivers
    Upland stream with browning of water from high concentrations of dissolved organic matter. Photo: Don Monteith

Lake and stream waters in upland regions of many parts of northern Europe and North America have become browner in recent decades, an issue of major concern for water companies who draw on upland drinking water sources. Now a team of scientists has developed a simple mathematical model that shows why these changes have happened, and how climate change might be an influence in the future.

The progressive browning of lake and stream water in upland regions reflects a rise in concentrations of dissolved organic matter (DOM). We normally quantify the amount of DOM through the measurement of dissolved organic carbon (DOC). Much of this DOM is produced from the decomposition of vegetation which washes into surface waters after heavy rain. The rate of decomposition is affected by temperature, so concentrations in streams often peak in late summer to early autumn.

By comparing trends in DOC across these upland regions, we can see that they strongly relate to reductions in indicators of air pollution in the waters, such as the concentration of sulphate. The comparison suggests that improvements in soil chemistry, resulting from reductions in air pollution, are resulting in organic matter in the soil becoming increasingly soluble, leading to more DOM leaching into surface waters. But questions have remained over the precise mechanism causing this effect, as well the role of rainfall and temperature variations in influencing change.

Why is this important?

Changes in DOM concentrations have a number of important implications for people and the environment. Upland surface waters provide a substantial proportion of public drinking water supplies in many regions, including the UK. Before water is released for consumption by the UK public it has to undergo disinfection, normally using chlorine. However, chlorination of water with excessive levels of DOM can result in the formation of potentially toxic by-products. It is therefore also often necessary to remove DOM from the “raw water” using various, and often expensive, water treatment methods.

Dissolved Organic Matter is also a source of plant nutrients such as phosphorus and nitrogen. DOM increases therefore have the potential to stimulate the growth of “nuisance algae”, which can clog filters at water treatment plants and influence the taste and smell of the water. Water companies have thus been facing mounting water treatment costs, in some cases needing entirely new treatment apparatus. They need better understanding of the processes behind the DOM increases, to reduce uncertainty over future treatment costs and infrastructure needs.

The browning of water has other ecological implications, such as a reduction in the depth and types of habitat in lakes at which aquatic plants grow, potentially affecting the ability of lakes to capture and store carbon.

Understanding the issue is also important for those developing Earth System Models. The movement of organic matter from the land through rivers and lakes into the sea is a major part of the global carbon cycle, comparable in magnitude to the amount of carbon captured by photosynthesis by terrestrial plants.

Our findings should therefore be of interest to water companies dependent on upland surface waters as sources of drinking water, global climate modellers who need to estimate rates of transfer of carbon from the land to the oceans, and aquatic ecologists concerned with impacts of long-term environmental change on upland freshwaters and biodiversity.

What we did

We developed a very simple mathematical model to simulate how DOC concentrations varied and changed over the period 1990-2020 in eight intensively monitored headwater streams in the UK, Czechia, Norway and Sweden.

In particular we wanted to test a recent theory that a reduction in the ionic strength of soil water (that is, a lower concentration of electrically-charged atoms caused by a reduction in the deposition of air pollutants) has increased organic matter solubility, leading to the long-term changes in DOC. We also needed to represent effects of variation in stream flow and air temperature to avoid the risk of wrongly attributing the pollutant effects in our model. For the UK catchments we combined DOC data collected by the UK Upland Waters Monitoring Network, precipitation chemistry provided by the UKEAP network Precip-Net and NRFA stream discharge data.

By using chemical measurements of locally collected rainfall, we showed that DOC responded to reductions in ionic strength in a remarkably consistent manner. The more rapid the dilution of rainfall over time, due to less polluted air, the more rapidly DOC concentrations increased. Over shorter time periods, however, much of the DOC variation could be explained by fluctuations in stream flow, while changes in temperature controlled more regular seasonal variation.

What we concluded

The industrial revolution and associated increase in the use of fossil fuels (coal and oil) that began some 200 years ago, resulted in a progressive increase in the emission of sulphur, hydrogen chloride and nitrogen to the atmosphere, and the phenomenon of “acid rain”. Since the 1970s, a series of internationally agreed emission treaties has resulted in huge reductions in pollutant deposition, and an accompanying reduction in the ionic strength of precipitation and water flowing through upland soils. Rainwater in much of the UK uplands is now probably more dilute than it has been for most of the last two centuries and is beginning to stabilise. Our modelling suggests, however, that DOC/DOM levels will continue to increase at least until this dilution process fully flattens out.

In the longer-term, our modelling implies that increases in air and soil temperatures and changes in precipitation patterns driven by regional climate change will increasingly dominate future DOM trends and determine the extent to which terrestrial carbon is exported from rivers to the oceans. Although warming predicted by current climate projections is likely to have relatively subtle effects, waters draining peatlands will be most sensitive and could experience unprecedented levels of water colour and treatment requirements in the near future.

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