20 Wetland Assessment Criteria 



replenishing aquifers used for water supply. Wetlands demonstrated to 

 be groundwater discharge sites are good indicators of potential water 

 supplies for towns. More work is needed to adequately understand this 

 function in specific wetlands (Sather and Smith, 1984). 



The effectiveness of the groundwater supply function of wetlands is higher 

 when the surface and groundwater aquifers are connected. The socio- 

 economic value is higher when the public derives its water supply from the 

 wetlands or related groundwater aquifer. The public benefits of this 

 wetland function include water supply for public use, irrigation, livestock 

 watering and wildlife uses. 



(ii) Water Quality 



Wetlands can help maintain water quality or improve degraded water by 

 removing, transforming and retaining nutrients; processing chemical 

 and organic wastes and pollutants (including heavy metals); and 

 reducing sediment loads. Wetlands intercept runoff from uplands before 

 it reaches the water and help filter sediments, nutrients and wastes from 

 flood water. It is important, however, to recognize that wetlands have a 

 finite capacity to perform this function. 



Important water quality functions of wetlands include uptake, 

 transformation and addition of materials as water flows through the 

 wetlands. Wetlands act as sediment, toxic substance and nutrient traps 

 and perform functions similar to a waste treatment plant. The waste 

 treatment or water quality improvement process occurring in wetlands 

 still needs additional study to understand retention mechanisms and 

 capacities. Wetlands also have an important water quality role as 

 sedimentation basins. Wetland vegetation filters (e.g., lowers turbidity 

 of floodwater) and holds sediments which otherwise enter lakes, streams, 

 reservoirs or harbors, often necessitating costly maintenance dredging 

 activities. However, excessive sedimentation may raise the elevation of 

 wetlands and accelerate their conversion to uplands, thereby eliminating 

 values for trapping sediments. Wetlands also assimilate toxic 

 substances, such as heavy metals and pesticides. The pollutant trapping 

 function can result in serious problems for fish and wildlife, e.g., 

 Kesterson National Wildlife Refuge and other refuges in the West collect 

 irrigation return flow water containing leached salts and other minerals 

 in toxic concentrations. 



The water quality value of wetlands is highest when there is a net removal 

 or detoxification of materials that would lower water quality further 

 downstream. As would be expected, wetlands in urbanized and 

 agricultural environments have more eutrophic water (i.e., excessive 

 amounts of dissolved nutrients that may stimulate biological growth and 

 reduce oxygen levels in water) than ones in forested and/or naturally 

 vegetated areas. 



