Return Flows estimate from Topnet for Modflow Use

[ChristinaB]

Return flows are assigned for each user and each user has a return flow type, source, and associated rate. Public water system return flows estimated in the WRIA 1 Water Management Phase III Task 4 (Tarboton, 2007) work for each drainage in the Lower Nooksack were used as appropriate.

Return flows in Topnet 2007 for self-supplied residential water use were set to 0.05 or 5% of the withdrawal was returned to the surface water system.

Return flows from lawn and garden irrigation are estimated at about 15% (Oad et.al, 1996) and return flows from inside use can be as much as 85%. The combined return flow is around 50% for the summer season and 85% in the winter. Because Topnet can only accommodate one return flow value, 50% was assigned to avoid overestimation of flows during the low flow season.

The source of water for the self-supplied or small public water system supplied commercial establishments is predominantly groundwater. Return flows in the Topnet 2007 were set to 0.1 or 10% of the withdrawal returned to the surface water system. The majority of commercial enterprises tend to primarily inside water use with some landscaping irrigation. The same 50% return flow value was assigned to commercial businesses as domestic water use. Further investigation of the return flows from specific commercial interests could be addressed in future work.

Return flows from the municipal entities, as noted in Table 5, totals 4.68 cfs. The total water use withdrawn for municipal sources is 4.51 cfs (the difference is -0.17 cfs). The explanations for the negative difference is attributed to the City of Everson treating the City of Nooksack’s wastewater and the City of Ferndale treating wastewater from Central City (114 residential connections serving 225 people), Belfern (24 residential connections serving 60 people), and Northwest Road (8 residential connections serving 20 people) public water systems as well as from residences on private wells. Conversely, some residences on city water are on septic systems and not contributing to return flows. Stormwater and groundwater contributions to the sewer flow (infiltration and inflow) account for the remaining difference between the water delivered and the wastewater treated in the non-irrigation season for the City of Ferndale.
In addition, Ferndale treats leachate from RECOMP’s and Olivine Corporation’s closed ash landfills as well as leachate from the Cedarville landfill. The generation of leachate is caused principally by precipitation percolating through waste deposited in a landfill. Once in contact with decomposing solid waste, the percolating water becomes contaminated and if it then flows out of the waste material it is termed leachate. The City of Ferndale’s 2011 Draft Sewer Comprehensive Plan Update states that the leachate from these sources is small. For purposes of modeling, the annual average return flow divided by the annual average diversion for each utility was used for developing the return flow inputs; distribution on a monthly basis is not possible in Topnet-WM. The return flow is represented as a percent of the diversion and the values can be found in Table 6. Aside from the City of Ferndale, the PUD diversions used outside the Lower Nooksack Subbasin have return flows equivalent to zero.

[ChristinaB]

ReturnFlow.txt

[ChristinaB]

Format_user.pdf

[ChristinaB]

Format_ReturnFlow.pdf

[ChristinaB]

user.txt