The Thames WaterBlitz was a citizen science event, led by Earthwatch and funded by Thames Water, in which volunteers tested rivers, streams and ponds across the Thames Valley for nitrate and phosphate over a few days twice a year from 2015; in the three events from 2019 to 2021 with a published share, 68% to 75% of samples showed high nitrate.
What volunteers measured
Each volunteer tested two nutrients, nitrate and phosphate, with simple colour-change tests, and recorded what they could see: the surrounding land use and whether algae was present (Earthwatch). The 2020 report counts a nitrate reading over 2 mg/l and a phosphate reading over 0.1 mg/l as high (Thames WaterBlitz report 2020), while the autumn 2019 report used higher thresholds, over 5 and 0.5 mg/l, so shares from different years are not directly comparable.
The results from 2015 to 2019
By spring 2019 volunteers had collected 2,796 water samples since the first WaterBlitz in 2015, and that spring 68% of samples had high nitrate while 71% had low phosphate (Earthwatch, spring 2019). In spring 2018, 20% of the rivers and streams sampled showed elevated nitrate (Earthwatch, 2018). The autumn 2019 event, from 20 to 23 September, drew 276 volunteers in the Thames Valley who collected 432 measurements, almost half of that event's total, and found litter at 14% of the Thames sites (Earthwatch, autumn 2019).
The results in 2020 and 2021
The tenth event, from 25 to 28 September 2020, was the largest: 879 volunteers recorded 966 measurements across the Thames Valley, close to 75% of water bodies showed high nitrate, about 75% of ponds had low phosphate, and freshwater shrimp were found at all but one of 59 locations surveyed for invertebrates (Thames WaterBlitz report 2020; Earthwatch, 2020). In spring 2021 the Thames Valley had the largest share of samples with high nitrate of any region taking part, about 70% (Earthwatch, spring 2021), and in autumn 2021, from 24 to 27 September, 355 volunteers recorded 513 measurements.
Taking part now, and the data
The WaterBlitz now runs across the UK as the Great UK WaterBlitz: the September 2026 event has finished and Earthwatch says the next one is in April 2027, and in its spring 2026 event 57% of UK data points showed poor water quality (Earthwatch). The Thames data from 2015 and 2016 is still published as public map layers, 629 points from 2015, 334 from spring 2016 and 252 from October 2016, and the FreshWater Watch dataset as a whole can be downloaded from freshwaterwatch.org.
Testing the water in your own home
The nutrient a river test and a drinking water test share is nitrate. In the United States the legal limit for public drinking water is 10 mg/L of nitrate measured as nitrogen, because infants under six months who drink more can become seriously ill (EPA). A private well is tested by its owner: our answers on removing nitrate from water and home water test kits cover what a household can do.
Questions people ask about thames waterblitz
What is the Thames WaterBlitz?
A citizen science event led by Earthwatch and funded by Thames Water in which volunteers tested Thames Valley rivers, streams and ponds for nitrate and phosphate over a few days, twice a year from 2015.
What did the Thames WaterBlitz find?
High nitrate in most samples: 68% in spring 2019, close to 75% of water bodies in autumn 2020 and about 70% in spring 2021, with phosphate low in most ponds.
Can I still take part in a WaterBlitz?
Yes. Earthwatch runs the Great UK WaterBlitz across the country; the next event after September 2026 is in April 2027.
Is the WaterBlitz data public?
Yes. The early Thames events are published as public map layers, and the FreshWater Watch dataset can be downloaded from freshwaterwatch.org.
Sources
- Earthwatch: WaterBlitz results, autumn 2020 (retrieved 2026-10-21)
- Thames WaterBlitz report, autumn 2020 (retrieved 2026-10-21)
- Earthwatch: Water Blitz 2019 (post text) (retrieved 2026-10-21)
- Earthwatch: autumn 2019 results (post text) (retrieved 2026-10-21)
- Earthwatch: Water Blitz results, 2018 (retrieved 2026-10-21)
- Earthwatch: spring 2021 WaterBlitz results (retrieved 2026-10-21)
- Earthwatch: the Great UK WaterBlitz (retrieved 2026-10-21)
- EPA: National Primary Drinking Water Regulations (retrieved 2026-10-21)
- WaterBlitz autumn 2019 report (archived) (retrieved 2026-10-21)