Phosphate in rivers

A case study

Too much of anything is bad

Phosphate is a natural nutrient in freshwater ecosystems such as rivers and lakes. Yet, phosphate pollution can lead to excessive algal growth, known as ‘algal blooms’.

Algal blooms can cause low oxygen levels killing fish and other organisms, and can release toxins harmful to humans and animals. The effects of nutrient pollution are estimated to cost the US economy alone $4 billion per year [1].

Therefore, nutrient levels in water need to be measured and managed. The EU Water Framework Directive and United States Clean Water Act seek to address this problem. However, widespread monitoring of phosphate has so far proved difficult.

The challenge of continuous monitoring

Typically, if you want to monitor phosphate, you collect water samples manually, transport them to the lab, and analyze them there. This procedure is very labour and resource intensive, and it often results in sparse 'low-resolution' data. How often can you send a staff member to collect a water sample?

Low-resolution data may miss important pulses, such as rain runoff, and hence make it difficult to understand the sources and variability in phosphate pollution.

Phosphate is often measured using colourimetric methods, where the water sample is mixed with colour-forming reagents. The intensity of the colour produced is proportional to the phosphate concentration in the sample.

Some in-field phosphate analysers exist (using automated colourimetric methods), but these are either large/bulky, require mains power, or consume large quantities of fluid per sample. High fluid consumption results in large quantities of chemical waste left over from the analysis, and we think it is important that this is not released into the environment.

The solution

The ClearWater Sensors phosphate sensor uses microfluidics and lab-on-chip technology: colourimetric phosphate analysis is conducted on a miniaturised, low-power device that uses very low sample volumes (e.g. 0.5 mL per sample).

It can produce laboratory-quality data without the drawback of high power consumption, so can operate on batteries in remote locations.

Liquid waste from the chemical anlaysis is produced in such low volumes that it is stored onboard the sensor in a user-swappable reagent canister.

The sensor can conduct over 2000 measurements per deployment and operates without mains power. It can be submerged for discrete, in-stream deployment and is readily moved to new locations to map variations. It can operate in either freshwater, estuaries, or the ocean.

River monitoring application

In this study, an early prototype of the ClearWater Phosphate Sensor (originally developed by the National Oceanography Centre - NOC) was used by NOC scientists to conduct automated high-resolution phosphate measurements in a chalk-fed river in the southern United Kingdom [2]. The Hampshire Avon is located in an area of phosphate concern, with anthropogenic phosphate sources including agricultural fertilisers, animal waste and discharge from sewage treatment works.

The sensor was submerged 1 m below the surface, hung from a bracket extending out from the riverbank. No mains power was available in this remote location, so the sensor was powered by a 12 V battery and solar panel located on the riverbank. Data was telemetered using 3G so that it was accessible in real-time via a web portal. A multi-parameter sonde was deployed alongside the phosphate sensor to collect ancillary data on water temperature, conductivity and dissolved oxygen.

The sensor was deployed for 9 weeks, with occasional visits by the scientists to collect grab samples for comparison purposes.

Data for new insights

The graph shows a snapshot of the data showing diurnal variation in phosphate over three days. With the full 9-week data set [2], scientists were able to plot detailed hysteresis relationships between river discharge and phosphate concentration, helping them to identify sources of phosphate in the catchment.

Please contact us at info@clearwatersensors.com for more information about how the ClearWater phosphate sensor can be used for your application.

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References

[1] Walter K. Dodds, Wes W. Bouska, Jeffrey L. Eitzmann, Tyler J. Pilger, Kristen L. Pitts, Alyssa J. Riley, Joshua T. Schloesser, and Darren J. Thornbrugh, Eutrophication of U.S. Freshwaters: Analysis of Potential Economic Damages, Environmental Science & Technology 2009 43 (1), 12-19

[2] Geraldine S. Clinton-Bailey, Maxime M. Grand, Alexander D. Beaton, Adrian M. Nightingale, David R. Owsianka, Gregory J. Slavik, Douglas P. Connelly, Christopher L. Cardwell, and Matthew C. Mowlem, A Lab-on-Chip Analyzer for in Situ Measurement of Soluble Reactive Phosphate: Improved Phosphate Blue Assay and Application to Fluvial Monitoring, Environmental Science & Technology 2017 51 (17), 9989-9995