Essex Rivers Hub

River Blackwater Catchment Overview

This catchment consists of the River Blackwater and Domsey Brook. The Blackwater is 61 km long and the Domsey Brook is a tributary that flows into the Blackwater and is 7km long.

The River Blackwater has an overall Water Framework Directive (WFD) classification of poor while Domsey Brook has a slightly better classification of moderate. The Blackwater is failing on diatoms, (plants not visible to the naked eye), phosphates (both have a WFD classification of poor), Macrophytes (plants visible to the naked eye) and morphology (both have a WFD classification of moderate). The Blackwater is failing for annex 10 chemicals, specifically the PAHs Benzo (ghi) perelyene and indeno (123-cd) pyrene, these chemicals are related to combustion processes and difficult to eliminate from the water course. The Domsey Brook is failing on phosphate which has a WFD classification of moderate.

The River Blackwater has good fish and aquatic invertebrate populations and has low levels of ammonia and high levels of dissolved oxygen. Domsey Brook has good fish population, good flows, high dissolved oxygen levels and low ammonia levels.

The reasons for diatom failure within the Blackwater are linked to increase nutrient enrichment within the aquatic environment as is the failure for macrophytes. The main cause of this is the high levels of phosphate. The main source of phosphate within this water body is point sources such as sewage treatment works and discharges from private residences and businesses. Decisions need to be made to determine the best methods to counteract there impact. The cause of the phosphate failure in the Domsey Brook is unknown so further investigation is needed, however as this brook is a tributary of the Blackwater it is likely that this water body is also impacting on the phosphate levels within the Blackwater.

The PAH pollution is mainly entering the watercourse from the sewage treatment works (not directly from actions within the works but something that isn't currently removed from the waste water) therefore it needs to be determined what actions are needed to tackle it at this source. Local action could be local environmental campaigns to reduce this pollution and raise awareness of the impact and determining effective treatments that can be carried out within the sewage treatment works to remove these chemicals. Another source is road runoff so SuDS schemes could be introduced to reduce the impact from this source.

It is crucial to note that as part of their ongoing efforts to prevent the pollution of river water, the Environmental Agency considers the presence of certain pesticides and herbicides in this catchment, to be an issue of concern and must therefore be carefully managed through Safeguard Zone Action Plans to protect raw water sources used for drinking water.

  • Metaldehyde - An organic pesticide used against gastropods in both household and agricultural settings. It is mildly toxic when consumed by other organisms, especially predators of gastropods (hedgehogs, frogs, wild birds), and has been linked to their recent decline. Read more.
  • Carbetamide and propyzamide - These are herbicides used in winter oilseed rape and winter bean fields which target various annual grasses and broad-leaved weeds. They pose no known risk to mammals, birds and bees but are highly toxic to aquatic organisms.
  • Nitrate - Comprised of oxygen and nitroten, this compound derives primarily from fertilisers and usually finds its way into groundwater and surface water via surface run-off. The UK and EU standards stipulate that the upper limit for contamination is 50 parts per million in drinking water and 50 parts per billion in fertilisers. Various water companies are currently working on plans to create modern nitrate removal plants to further the measures to protect drinking water. 
  • Clopyralid - A selective herbicide which internally disrupts various broad-leaved weeds, causing plant death. It is relatively persistent in soils but is generally of low toxicity to mammals, birds, aquatic organisms and other non-target organisms. 

 

Local water companies will continue to strive to abide with EU regulations as studies have shown that the above compounds pose a low risk to human health in small doses. Despite this, many mitigation measures have been proposed for this catchment. Currently both water bodies have sediment management strategies, invasive species management and selective vegetation control in place. The River Blackwater also has mitigation measures to reducing dredging and its impact on this water body. Only one mitigation measure has not been put in place on the Domsey Brook but 8 mitigation measures are outstanding for the River Blackwater, the next step is to determine if these are still require and implement if necessary. The measures are as follows:

Blackwater mitigation measures not in place

  • Determine if still required.
  • Removal of hard bank reinforcement / revetment, or replacement with soft engineering solution
  • Increase in-channel morphological diversity
  • Bank rehabilitation / reprofiling
  • Improve floodplain connectivity
  • Structures or other mechanisms in place and managed to enable fish to access waters upstream and downstream of the impounding works 
  • Preserve and where possible enhance ecological value of marginal aquatic habitat, banks and riparian zone
  • Operational and structural changes to locks, sluices, weirs, beach control, etc.

Domsey Brook mitigation measure not in place

  • Appropriate channel maintenance strategies and techniques - minimise disturbance to channel bed and margins

 

 

 

 

 

 

 

 

 

 

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