River Chess: Real-time Water Quality Dashboard

What is pH?

pH is a measure of the concentration of hydrogen ions in water. pH range is from 0 to 14 with 7 being referred to as a ‘neutral’ pH. pH less than 7 indicates acidic conditions, whereas a pH greater than 7 indicates  conditions.

pH is measured on a negative logarithmic scale, so that a change in 1 unit of pH is equivalent to a ten-fold change in the concentration of hydrogen ions.

Why do we measure pH in river water?

pH controls many of the chemical reactions that will occur in the water and determines which aquatic life can use the water. Fish, for example, are particularly sensitive to changes in pH in a river.

Chalk rivers are naturally alkaline with a pH of between 7.4 and 8.7. Their pH is controlled by a combination of the geology, the plants in the river, and human activity. Pollution can change the water’s pH, which in turn, can harm the fish, animals and plants in the river.

How does geology influence pH?

Chalk is limestone made from the shells of ancient algae called coccoliths which lived approximately 100 million years ago. The chemical constituents of chalk are calcium and magnesium carbonate. This carbonate will react with hydrogen ions in any water that flows through it, so that water becomes alkaline in nature (i.e. has a low hydrogen ion content).

British Geological Survey have a useful online map of principal aquifers in the UK which provides further information on chalk geology.

How do aquatic plants control the pH?

The pH of chalk rivers is also controlled by the plants in the river (i.e. aquatic plants) on a daily (or diurnal) cycle. Aquatic plants both respire and photosynthesise. Plants respire all the time, but they only photosynthesise when they are in the light. Photosynthesis uses up carbon dioxide in the river water causing carbon dioxide concentrations to drop under bright light levels during the day, whereas respiration produces carbon dioxide so levels rise at night.

Carbon dioxide dissolves in water to produce carbonic acid, which in turn dissociates to release hydrogen ions. At night there is more carbon dioxide, and therefore more hydrogen ions in the river water, compared to the daytime. This means that water is more acidic at night compared to during the day.

Aquatic plants grow in the river from Spring through Summer, and die back in Autumn and Winter. Therefore, there are more plants photosynthesising and respiring in spring and summer compared to winter. This means that the plants cause greater daily changes in pH in spring and summer than can be seen in winter.

How can human activity change the pH?

The pH of a chalk stream can be altered if acidic runoff enters the river, and changes the river chemistry. In some parts of the UK this has occurred due to highly acidic mine water entering the river system. In the Chilterns it will be interesting to find out whether road runoff and/or inputs from sewage treatment works modify the pH of the river water.

What is dissolved oxygen?

Dissolved oxygen is a measure of the amount of gaseous oxygen (as O₂) dissolved in water. Oxygen enters the river water from the atmosphere (via diffusion), from groundwater , and also as a by-product of the photosynthesis of aquatic plants. Oxygen levels in water can be expressed in units of concentration, as mg/L, or in relation to the maximum amount of oxygen that can be dissolved in the water at a certain temperature, called ‘% saturation’.

Why do we measure dissolved oxygen?

Fish and zooplankton need oxygen in water in order to breathe. Good oxygen levels are critical for the health of a river system. Slow flowing, polluted river water is often associated with low oxygen conditions, which cannot support much life. As we will see dissolved oxygen is closely linked to organic pollution and nutrient enrichment, so can tell us quite a bit about the health of a river.

What are the natural controls on dissolved oxygen?

Oxygen levels in river water are a balance between oxygen supply (from the atmosphere and by photosynthesis) and oxygen consumption due to the respiration of plants, animals and microbes. Bacteria use organic matter (including plant and animal remains) in water as a food source. The more organic matter, the greater the number of bacteria respiring in the water, and hence the more oxygen that gets used up by the bacteria. Therefore, excess organic matter in rivers can cause low oxygen conditions.

The solubility of oxygen in water decreases as water temperature increases. This means that oxygen levels can show seasonal variations, with less oxygen in summer when water temperature are high, and more oxygen in winter when temperatures are low. This can have important consequences for fish health.

Chalk streams are valued for their aquatic plants which grow over the spring and summer months. These plants will photosynthesise by day, and respire at night causing marked daily changes in the dissolved oxygen content of the river. The rate of photosynthesis will change depending on the strength and duration of sunlight, so a sunny day will be associated with higher rates of photosynthesis compared to a cloudy one.

How can human activity change dissolved oxygen?

Sewage (also called domestic wastewater) contains human faeces which is made up of organic matter. One of the purposes of sewage treatment is to break down the organic matter under controlled conditions so that the organic matter is not released into the river where it can cause harm to people and river life. Any release of raw sewage into a river is accompanied by a decrease in dissolved oxygen content because bacteria, which use the sewage as a food source, grow in number and respire. This respiration rapidly uses up oxygen in the water and can lead to fish kills.

In the UK we have combined sewer systems which collect both stormwater from roads, roofs of buildings and car parks when it rains, as well as sewage from our homes and businesses. When rainfall is heavy, hard surfaces in the catchment rapidly transport water to our sewer systems. Overflows from sewers into rivers can occur if there is not enough capacity in the sewage treatment works to handle these stormwater flows.

Misconnections in the underground pipe network can also mean that sewer pipes are incorrectly linked to overflows to rivers. These misconnections can be quite common in built up areas, and result in water high in organic matter reaching our rivers which can cause problems with oxygen levels.

What is turbidity?

Turbidity is a measure of the cloudiness of water, quantified by measuring the amount of light that is scattered by a river water sample when light is shined through it. Turbidity is, therefore, an indication of the amount of particulate material that is being carried by the water (such as clays, silts and algae). Turbidity is measured in ‘Nephelometric Turbidity Units (NTUs)’.

Why do we measure turbidity in rivers?

The water in chalks streams is usually very clear and transparent, because chalks streams receive their water predominantly from the ground, and the water is filtered as it moves through the chalk. However, if the water does become cloudy due to transport of sediment there are several problems that can occur. Chalk streams are valued for their aquatic plants, and these plants growing in the rivers need good light conditions. Cloudy water stops light passing through the water to the plants and affects their growth. Chalk rivers are also home to many fish, such as trout, and these fish lay their eggs in the riverbed. Sediment that is transported in the water can settle on to the riverbed and smother these eggs preventing them from developing and hatching out into small fish.

What are the natural controls on turbidity in rivers?

The geology and soils in a river catchment is a key control on turbidity. If a river has its source in an area of clay soil, which is easily transported by water, then river water is likely to be cloudy. Chalk streams are fed by groundwater from chalk aquifers. This groundwater has been naturally filtered by the chalk and contains very little sediment and particles. Consequently it is very clear.

How can human activity change turbidity?

Elevated turbidity or cloudiness can be caused by sediment entering the river from urban and/or agricultural activities. In urban areas, cloudiness is often associated with intense rainfall when solid material that has collected on hard surfaces can be washed into the river, or where mis-connections with sewers and drainage occur. Unfortunately, sediment can also transport pollutants, such as metals, which are attached to the particles. These pollutants can also affect the health of the river. In agricultural areas, cloudy water can be associated with loss of soil especially from land with little or no vegetation cover, and often on hillslopes close to the river. Farm animals that trample the river banks can also cause soil to enter the river, and cause cloudy water.

What is water level?

Water (or river) level is a measure the depth of water in a river at a specific location. Scientists also refer to the measurement of the level of water in a river relative to an arbitrary point (e.g. the river bed) as ‘river stage’.

Why do we measure water level?

Measurements of water level in a river are important for a variety of reasons. In the UK the Environment Agency operate a network of monitoring stations that continuously measure river level primarily to understand, and provide a prior warning of, flood risk to people and properties. On the River Chess there is a continuous water level monitoring station at Rickmansworth.

River water level is also important for the health of a river as many plant and animal species need specific ranges of water level at different times of the year. We also use water level to calculate how much water is flowing through a river per unit of time (known as ‘discharge’).

What are the natural controls on water level?

In many rivers the water level responds quite quickly (within hours) to rainfall falling in the river catchment. In a chalk catchment a significant proportion of the rain will move through the ground into the underlying chalk aquifer, and this rain can take decades to reach the river. This means that water levels and discharge in a chalk river have a distinctive pattern that relates to the time of year or season.

Chalk streams are particularly interesting because sections of these rivers (termed winterbournes) can, quite naturally, stop flowing at times of the year when groundwater levels are low. This is because groundwater supplies these rivers with much of their flow. When groundwater levels drop then flow in the river will decrease and may even cease.

Here you can see how the annual water cycle works in a chalk aquifer.

How do human activities change water level?

Humans abstract (remove) water from rivers to use in our homes, farms and industries. In areas of the UK with a chalk geology we often extract water from the ground (i.e. groundwater) for domestic water supply. Because chalk rivers depend on groundwater for their flow (> 75% of the water in a chalk river comes from the ground) then using groundwater for domestic supply can reduce the level of water in the river. This means that stretches of the river can dry up at unexpected times in the year, or for longer periods of time than might be desirable or healthy for fish.

Much of the water we use in our homes travels through our drainage systems to the local sewage treatment works. This water is treated to remove harmful chemicals, and then discharged into the nearest river as ‘treated effluent’. Because human populations are high in the South East of the UK, and water usage is also high, this treated effluent can make a very significant contribution to river levels, flow and water quality. For example, in the River Chess treated sewage effluent can comprise up to X % of the river water, depending on location.

Humans also affect the speed and extent to which water levels can change in our rivers. When we create hard surfaces such as roads and pavements we alter the flow of rain water which moves over our street surface and into drainage pipes, instead of through the soil to the groundwater below. This flow of water is called ‘urban runoff’. This rapid flow of runoff means that water levels in chalk rivers with an urban catchment will respond more quickly to rainfall than might occur naturally. Also, only a little water is lost to the ground, so a greater volume of water reaches our rivers. Urban runoff may carry with it the pollutants that have been deposited on the road and pavement surfaces (e.g. metals and oils).

What is electrical conductivity?

Electrical conductivity measures the ability of water to conduct an electrical current. The higher the concentration of dissolved charged chemicals (also known as salts) in the water, the greater the electrical current that can be conducted. Examples of charged ions that naturally occur in river water include calcium, potassium, chloride, sulphate and nitrate.

The higher the temperature of the water, the greater the ability of the water to conduct electrical charge. For this reason electrical conductivity is always reported at a reference temperature of 25 ^oC. The unit of measurement is microsiemens per cm (μS/cm). Electrical conductivity in a river can be quite variable, and still within natural levels that will not cause any harm. Typical values for a chalk river will be 100 – 2000 μS/cm.

Why do we measure electrical conductivity in rivers?

Significantly elevated electrical conductivity can indicate that pollution has entered the river. A measure of electrical conductivity cannot tell you what the pollutant is, but it can help identify that there is a problem that may harm invertebrates and/or fish. Electrical conductivity may be high in a river without any visible effects on the clarity of the river water, so it needs to be measured with a suitable sensor.

What are the natural controls on electrical conductivity in rivers?

Geology and soil type are the main natural controls on electrical conductivity in rivers. When it rains, water flows through and over the soil to our rivers, dissolving and picking up chemicals along its way. In regions underlain by chalk, rainwater will move into the soil, and then flow down to the chalk underneath. As water moves through the chalk it will dissolve magnesium and calcium carbonate, which raises the electrical conductivity. It generally takes decades for the water to move through the chalk to the river, so there is plenty of time for the water to dissolve the chalk. The residence time of water in rocks and soils is an important control on electrical conductivity.

How can human activity change electrical conductivity?

Any human activity that adds inorganic, charged chemicals to a river will alter the electrical conductivity. For example, electrical conductivity may be higher in a river downstream of a sewage treatment works due to chemicals such as chloride and phosphate from household products. Winter road runoff, containing salt, can be very high in electrical conductivity. If this runoff reaches rivers then it may, depending on the quantity of water, temporarily elevate the electrical conductivity in the river.

What is chlorophyll-a?

Algae are aquatic organisms containing chlorophyll within their structure. Algae live in the water column (called phytoplankton) and attached to the river bed. They are an important food for other river life such as small animals and freshwater shrimp.

The chlorophyll molecule allows algae to absorb energy from light; a process known as photosynthesis. Thus chlorophyll can be used as a measure of algal content in rivers. Chlorophyll-a is a type of chlorophyll molecule which is common in algae. Whilst measurement of the chlorophyll-a content of river water will not measure all of the algae in a river, it can be a good overall indicator of general patterns in phytoplankton growth and die-back and is widely used by freshwater and marine scientists.

Why do we measure chlorophyll-a?

Algal growth is often linked to nutrient enrichment in freshwaters. Whilst algal growth is a natural occurrence, excessive algal growth can be a symptom of eutrophication. Much of the work on eutrophication has been carried out in lakes, but we can apply some of these findings to the river environment. By day algae will photosynthesise and produce oxygen. However, when excessive algal growth occurs, then the decay of this algae (which is essentially organic matter) will be accompanied by the rapid growth and multiplication of respiring bacteria which may use up dissolved oxygen in the river water.

In chalk streams filamentous algae can grow on the river bed smothering the gravels, preventing good light penetration to aquatic plants, and hence affecting their growth.

What are the natural controls on chlorophyll-a?

In addition to nutrient levels, light levels are also a critical control on algal growth. The clear water of chalk streams favour growth of algae, and there can be some clear differences in algal growth in areas of the river that are shaded by trees compared to those river reaches where there is sunlight throughout the day. A combination of high water temperature, good light levels and slow flowing water (so that there is time for algae to grow) are thought to favour algal growth. Consequently in UK rivers there are often sustained period of algal growth, termed ‘algal bloom’, from Spring into early Summer. Sometimes a second algal bloom occurs in late Summer and early Autumn.

How can human activity change chlorophyll-a?

Algae need nutrients to grow, but high levels of nutrients can cause algal blooms to occur. Unfortunately human activity can cause nutrient levels in rivers to rise, and this has been a long-term problem in UK rivers for several decades. Two important nutrients that have been elevated in our rivers are nitrogen and phosphorus. High nitrogen concentrations occur for a variety of reasons including agricultural inputs from fertiliser application (including slurry and man-made fertilisers) and treated sewage effluent. The relative importance of the different sources of nitrogen depend, to a large extent on the type of farming, and human population in the catchment. Phosphorus also enters our rivers from fertiliser applications, and from household cleaning products which find their way to our sewage treatment works via our drainage systems.

What is tryptophan?

Tryptophan is an amino acid that is present in many foods that are rich in proteins. Amino acids are organic molecules that are the building blocks of proteins.

Tryptophan fluoresces in the ultra-violet region of light. This means that when you shine a light of a particular wavelength on the molecule, it will emit light of a longer wavelength. For example, if you shine light of 280 nm wavelength on tryptophan it will emit light at 340 nm wavelength, and the intensity of this emitted light can be detected and measured by a suitable probe. ‘nm’ is short for nano-meter which is one billionth of a meter in length (1 x 10^-9).

Why do we measure tryptophan?

Some types of organic matter also fluoresce at the same wavelengths as tryptophan. ‘Tryptophan-like’ fluorescence in river water is associated with the presence of organic matter that can be easily degraded by microbes. This type of organic matter can arise in rivers from sewage inputs and from farm wastes such as cattle and pig slurry and silage. So the signal from a tryptophan probe can be used to identify the presence of these organic wastes in a river. This is quite a new technology, and the applications are still under development.

What are the natural controls on tryptophan?

Here are thousands of different types of organic matter molecules in a river made up of decomposing plants, animals and microbes. Because many types of organic matter fluoresce in the tryptophan region there will be temporal variations in the signal from a tryptophan probe, which provide a measure of the microbial activity that quite naturally occurs in the river.

How can human activity change tryptophan?

Because human sewage fluoresces at the same wavelengths as tryptophan, then we can measure accidental spillages of human waste into a river with a tryptophan probe. Such a probe might also help identify mis-connections with sewer pipes in urban river locations.

Farming activities can involve spraying organic fertilisers such as slurry onto field surfaces. Slurry has to be stored in containers under strict environmental regulations because it can be extremely harmful to the freshwater environment. An accidental release of slurry to a river can cause bacteria, which would use the slurry as a food source, to grow and rapidly multiply. These bacteria use up oxygen in the river water, causing dissolved oxygen levels to drop rapidly. This can lead to fish kills.