2.6. Pollution in Rivers#
Pollution in rivers has been a growing concern. In general, pollution in rivers is from the release of sewage, chemicals and nutrients into the rivers. The River Trust has a bunch of maps that track pollution levels in rivers across the UK, including the amount of ‘forever chemicals’ (e.g., chemicals that are very hard to degrade so will stay in rivers for a very long time) and the rate of release of sewage. However, the largest source of pollution to rivers is through nutrients from agriculture.
Nutrients and Eutrophication#
Nutrients refer largely to nitrate and phosphate, which are key macronutrients that are needed for photosynthesis. Nitrogen (in the form of ammonium or nitrate) is used in amino acids and proteins while phosphorus is used for DNA and cell walls and lipids. To get plants to grow you need a source of CO2, water, light, nitrogen and phosphorus. This is why nitrogen and phosphorus are often added to soils in the form of fertilizers to maximize plant growth. Currently UK farmers are not penalised for overfertilizing their land, so it is just the cost of the fertilisers that limits the amount that would be used to stimulate growth.
As you add nitrogen and phosphorus run-off from agriculture to rivers you get growth of algae in the rivers, often measured as the chlorophyll abundance (Fig. 2.46).
Fig. 2.46 The addition of nitrate and phosphate leads to exceptional algal growth, shown by chlorophyll abundance.#
And we know that more algal production leads to more respiration which depletes the water in oxygen. This is eutrophication.
Fig. 2.47 The process of eutrophication: excess nutrients lead to algal blooms, which increase respiration and deplete oxygen levels in the water.#
In the lecture we discussed the particular challenge of the River Wye, where intensification of chicken farming has led to mass eutrophication and fish kills along the river. Focusing on cleaning up rivers often focuses on stemming the tide of runoff into the river. But much of the waste is seeping into the groundwater. We can start to ask ourselves how long until it reaches the rivers and the challenges with eutrophication might be over?
Transit Time and Groundwater#
The transit time simply might be considered:
Where the length might be how far a farm is from the river of interest and the velocity could be the rate of flow of a stream or the rate of flow of the groundwater derived from Darcy’s Law. It has long been thought that time scales with length. Next week in Andy’s lectures, he is going to show us why – when it comes to groundwater systems – time scales with length2 (due to the flowpaths in the subsurface and the impact of dispersion).
Groundwater and Surface Water Interactions#
The take home from groundwater is the importance it plays in the overall surface water system!
Fig. 2.48 Diagram showing the network of interactions between groundwater and surface water systems.#