Storm water management approach to improve watershed quality

Point Source pollution within the watershed is fairly well identified and regulated with few options to further reduce inputs outside  investment in better technologies to remove contamination from water to be discharged into the system.  Because higher tech systems come at a higher cost it may be more effective for users to pay for removal of an equal or larger amount of nutrients from the system by installing natural storm water treatment features in urban and agricultural areas that will provide a better return on investment than trying the remove  the last 5 mg/L of a nutrient out of wastewater discharge point.  Generally nutrients from waste water can be reduced to around 10 ppm using low tech solutions, in order to achieve lower concentrations to meet TMDL’s  higher technology systems must be used.(DC Water, 2016)  Rather than making that investment the money could be better spent else ware in the watershed to remove a larger amount of nutrient from the system using natural filtration systems like a buffer strip or bio filtration system to treat storm water and reduce the volume and velocity in which the storm water enters the drainage system.

Buffer strip

http://photogallery.nrcs.usda.gov/netpub/server.np?find&catalog=catalog&template=detail.np&field=itemid&op=matches&value=732202&site=PhotoGallery

Bio infiltration system

DC Water, (2016)   Nitrogen Reduction program,  Accessed from: https://www.dcwater.com/education/nitrogen_reduction.cfm#existing

  

The benefits of developing green infrastructure are many and reach beyond water treatment  they include :

·      Reduced runoff quantity

·      Longer resonance time of water in the system

·      Habitat Improvement

·      Soil conservation

·      Improved livability  through green space

·      Enhance property value

·      Low impact and generally low cost

·      Promotes native planting and growth

·      Promotes stream restoration and health

(DC water 2015)  DC Clean Rivers Project, Green Infrastructure Program, Power point presented 19 October 2015,  accessed from https://www.dcwater.com/education/gi-images/2015-0921%20ANC%203C%20Presentation.pdf

For urban areas I am recommending the use of bio filters and water gardens to reduce the volume and velocity of the runoff. Because parts of the watershed are underlain by karst topography it will be important to ensure that we are not loading groundwater in those areas and destabilizing the subsurface resulting in a sinkhole. That does not limit the use of bio retention and infiltration trenches, as long as they are lined in those areas.

Bio filters in a laboratory setting have been shown to remove up to 85% of phosphorus, 70% nitrogen and 95% of suspended solids from storm water prior to discharge for a properly sized unit.  (Bratieres et al, 2008) Although these results were observed in a controlled setting, a lower % removal would still make a significant impact in real world use given proper sizing and maintenance.

In urban areas the goal is to treat and address storm water where it falls and to mitigate storm surges in the water system that often increase the sediment, nutrient, and trash in the water course. This approach has an additional  benefit of reducing  scouring of the local stream beds to allow for stream bank stabilization. A side benefit is a increase in quality of life in hares with a high % of impervious surfaces, which often coincide with poorer areas in municipalities.

The state of Maryland has several good examples of storm water management systems in their design manual: http://www.mde.state.md.us/programs/Water/StormwaterManagementProgram/MarylandStormwaterDesignManual/Documents/www.mde.state.md.us/assets/document/chapter3.pdf

Buffer strips

For both urban and agriculture areas but especially for the agricultural regions of the watershed, we should promote the use of vegetative buffer strips  to protect streams weather herbaceous ( grasses and/or brush) or forested.  Regardless of the type of buffer there is general consensus  among  practitioners that a minimum 35” buffer on either side of a waterway is a preferred for an effective buffer strip to have the greatest effect but as seen in the North Carolina State report on buffer use and effectiveness, even a 15 ‘ buffer would have a significant impact to stream and watershed health. ( NCSU, 2016)  There will need to be a bit of negotiation in how much land a famer or landowner is willing to give up which may or may not be the optimum thickness, but we should  be sensitive to what the minimum thickness required to make a significant difference and what can be implemented.

Buffer strip effectiveness

Buffer Type	Width (M)	% Sediment Reduction	% Nitrogen Reduction	% Phosphorus Reduction
Grass	4.6	61	4	28
Grass	9.2	74	22	24
Forest	19	89	74	70
Grass/Forest	23.6	96	75	78
Grass/Forest	29.2	97	80	77

( adapted from NCSU, 2016)

Thicker and forested buffer strips provide the best return on value for stream protection and nutrient removal. They have an added benefit of adding richness in the biodiversity of the watershed and protection for fields from wind and water action helping to keep soils and nutrients on the property.  Tree’s add to the quality of streams by reducing the water temperature through shading, and providing structure to the stream in the form of downed trees and snags within the waterway that provide habitat for many species.

Grass and shrub buffers are still beneficial to the health of the watershed and conserving nutrients and soil on surrounding landscapes through soil and bank stabilization and reduction of water velocity entering the stream system from overland flow.

Example of herbaceous buffer strip removal actions

http://www.soil.ncsu.edu/publications/BMPs/fig4.jpg

This all assumes that nutrient management in agricultural areas are fully in place.  Leaky or failing animal waste storage ponds and improper land spreading can often would overwhelm even the best natural buffer strips and should be a priority in areas where best management practices are not being applied.

The other elephant in the room is funding, who pays for all of this and how?

Reference

Bratieres, K. Fletcher, T. Deletic, A. Zinger, Y, (2008) Nutrient and Sediment removal by stormwater biofilters: A large-Scale design optimization study, Water Research, Vol 42, August 2008, pp. 3930-3940

NCSU (2016) Riparian Buffers: What are they and Why do they work?, North Carolina State University,  Accessed from: http://www.soil.ncsu.edu/publications/BMPs/buffers.html#definition