Freshwater Quality and Pollution

The quantity of global freshwater resources has been changing over time and human activity has been one of the largest drivers in this change. The sustainable use of freshwater resources has been one of the main themes that I have been exploring throughout my blogs in terms of ‘quantity’. However, another aspect of sustainable use can also include the ‘quality’ of freshwater resources, since polluted freshwater resources can limit the extent to which this resource can be consumed. In this blog, I will explore how agriculture, industry and urban waste impacts the quality of freshwater resources.

Freshwater resources and ecosystems are becoming more easily polluted, whereby waste from multiple sources (agricultural fertilisers, industrial chemicals, and urban/human untreated waste) are being deposited into freshwater systems, and are ultimately polluting these resources beyond natural levels (Zamparas & Zacharias, 2014). Eutrophication is a large concern in many freshwater ecosystems due to the increased deposition of phosphorous (P) and nitrogen (N) fertilisers and other polluting chemicals from both point and non-point sources.

Non-point sources include the global consumption of fertilisers in agriculture, and the poor management of fertiliser application increases the losses of these nutrients from the soil through surface runoff processes; 20% of nitrogen fertilisers is lost through surface runoff and leaching (Khan & Mohammad, 2014). The nutrient enrichment of freshwater bodies occurs where algal blooms develop over the surface of the water which results in a huge decline in the quality of the water, inducing a state of hypoxia – hypoxia occurs when dissolved oxygen levels fall below 2ml of O₂/litre, making it difficult for oxygen dependent plants and organisms to live in (Diaz & Rosenburg, 2008). An interesting study by Withers et al. (2014) explains that there is a delay during which the effects of applying inorganic fertilisers will appear. For example, in the UK the intense application of N and P fertilisers was encouraged during post-World War II period to produce more food for the nation. However, these nutrients were stored temporarily or permanently in the soil until runoff or leaching into groundwater resources occurred, and this left behind a ‘legacy’ of background leakage of nutrients in UK inland waters. Thus, nitrogen levels appear to be increasing across UK lowland aquifers, despite lower levels of nitrogen fertilisers being used today and the stores of fertilisers in the soil and groundwater provides sources of nutrients during periods have no runoff or leaching due to low rainfall (Howden et al., 2011). Hence, freshwater resources in the UK are expected to experience long-term declines in water quality from both historical and current uses of fertilisers.

Point sources include wastewater from industrial and treatment plants, whereby pathogenic organisms, inorganic and toxic chemicals contaminate local freshwater systems and environments. For example, Lake Geneva in Switzerland supplies water to 70,000 people, however Vida Bay in Lake Geneva is one of the most contaminated areas of the Lake due to wastewater contamination (Thevenon & Poté, 2012). In this study, Thevenon & Poté (2012) uses sediment cores to reconstruct the polluting elements over a decadal timescale from 1200 to present (Figure 1). The record shows that from 1900, trace metal elements have increased significantly, compared to the steady levels between 1200-1800 such as lead (Pb) from 20-30mg/g to 60mg/g from 1600 to 1960. The sediment record at Vida Bay (Figure 2) shows an increase in caesium (¹³⁷Cs) at ~45cm of the sediment record which coincides with the construction of the outlet pipe of the wastewater treatment plant in 1964 at Vida Bay (Thevenon & Poté, 2012). Hence, the discharge of treated industrial/domestic wastewater results in further contamination of the environment and aquatic ecosystems. Furthermore, the surface sediments which contains high organic matter contents (e.g. P and N) reflects faecal indicator bacteria, Escherichia coli – in 2007, high concentrations of E.coli of 10⁴-10⁶ CFU/g was located around the Vida Bay outlet pipe and 10⁵-10⁷ around Chamberone River compared to 1996 where levels were non-existent. Such increases in trace metals and faecal bacteria in Vida Bay will have adverse effects on human health due to contamination of drinking water; this case study highlights the significant role industries have on freshwater contamination in addition to the agricultural use of fertilisers.

Figure 1. Sedimentary trace elements from centre part of Lake Geneva (Thevenon & Poté, 2012)

Figure 2. Sediment record from Vida Bay in Lake Geneva (Thevenon & Poté, 2012)

Freshwater resources are also being polluted due to the poor management of wastewater in urban areas, and predominantly in rapidly urbanising cities that are unable to implement adequate sewerage and treatment facilities. For example, the per capita pollution load of urban discharge into the Bagmati River, Kathmandu Valley in Nepal is estimated at 31gBOD/capita/day (Kam & Harada, 2001). Biochemical Oxygen Demand (BOD) increased from 3.8 to 30mg/litre from 1995 to 1998, and faecal coliform increased from 1.0x10⁴ to 8.75x10³MPN/100ml in the same period. In Dhaka, BOD and faecal coliform levels are within 20-30mg/litre and 10⁴-10⁵MNP/100 range, but the environmental standards for safe drinking water are less than 3mg/litre for BOD and 5000MPN/100ml (Kam & Harada, 2001). Current levels in Dhaka exceeds the safe human consumption limit due to pollutants from various urban sources (domestic wastewater) which are being discharged unsafely into rivers and local water sources. 

Concluding Thoughts:

Freshwater resources and ecosystems are subjected to huge declines in water quality as a result of human activities, ranging from agriculture, industrial activity and urban living spaces. In addition to the over-consumption of freshwater resources, the degradation of freshwater quality can equally diminish the amount of freshwater available to us for human use. Overall, managing both the quality and quantity of freshwater resources are important in achieving water security, especially in a warming world where water resources are becoming more scarce. The application of fertilisers should be timed so as to avoid heavy rainfall periods, industrial wastewater should be better managed before discharging into the environment and urban wastewater and sewage systems should be at the forefront of urban policies. Freshwater quality can be easily maintained but will require human intervention and better management.