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HomeMy WebLinkAboutCORRESPONDENCE - PURCHASE ORDER - 9174670 (2)Proposal Estimation of Preliminary Effluent Limits (PELs) for the Mulberry Wastewater Reclamation Facility of the City of Fort Collins Relevant to CDPS Permitting for Hypothetical Discharge to Two New Discharge Points Prepared by: Western Environmental Analysts Prepared for: Jason Graham City of Fort Collins December 1, 2016 Introduction Planners for the Northern Integrated Supply Project (NISP) have asked the City of Fort Collins to consider possible changes in the location of effluent discharge for the Mulberry Wastewater Reclamation Facility (MWRF), which is owned and operated by the City of Fort Collins. In order to evaluate the feasibility and cost that the City might incur in accepting new discharge points for MWRF effluent, the City must be able to examine quantitatively derived hypothetical effluent limits for MWRF on the assumption that the discharge points are moved as suggested by NISP. Such limits are designated as preliminary effluent limits (PELs) by the Water Quality Control Division (WQCD). The PELs, when approved as valid by WQCD, can be compared with current plant performance characteristics for individual constituents that are regulated by permit for MWRF. The scope of work given here is for the estimation of PELs for the assumed hypothetical new discharge points for MWRF. Preparation of the PEL estimates requires that all protocols used in calculating effluent limits be the same as those routinely used by WQCD in permitting so that the PEL estimates are reliable indicators of the likely effluent limits that would be incorporated into a permit in the future, if the City should decide to accept the NISP proposal. Computation of PEL values for all relevant constituents is site-specific. The estimation project described here will provide two sets of PEL estimates, one of which is for a site on the Cache la Poudre River and the other for discharge to Timnath Reservoir via the Timnath Reservoir Inlet Canal. Factors that must be taken into account quantitatively for PEL determination include biologically based low flows (DFLOW values) for discharge to the Cache la Poudre River and estimation of dilution potential for effluent reaching the reservoir. In addition, upstream water quality must be evaluated quantitatively according to protocols that are 1 used by WQCD in preparation of permits. Upstream water quality for the discharge to Timnath Reservoir and for the new discharge point on the Cache la Poudre River will be estimated from water quality data collected by the City at an upstream monitoring location. Site Description Figure 1 shows the location of MWRF; its effluent discharge is located just below the treatment facility. Below the MWRF discharge point by a distance of approximately 0.5 mile is the Timnath Inlet Ditch, which diverts water from the Cache la Poudre (CLP) River through a canal (~5 miles long) to Timnath Reservoir. Just below the Timnath Inlet Ditch is the proposed new discharge point on the Cache la Poudre River. Effluent from MWRF also would, according to the NISP proposal, be permitted for discharge directly to the Timnath Inlet Ditch by a pipeline as shown in Figure 1. The effluent in the pipeline will mix with any other water en route to the reservoir. Under these circumstances, the discharge point for MWRF is to the reservoir rather than to the Cache la Poudre River, but the water entering the Reservoir consists of MWRF effluent only or effluent plus diversion water, depending on ditch operations. In summary, the NISP proposal is for two new discharge points: Timnath Reservoir via Timnath Reservoir Inlet Ditch, and CLP just below the Timnath Reservoir Inlet Ditch. Hydrology Hydrologic analysis for the PELs is based on a ten year record of flow for the Cache la Poudre River at the Lincoln gage. In addition, use of diversion records for Timnath inlet will be necessary in estimating DFLOW values for the downstream hypothetical discharge point on the Cache la Poudre River. 2 Figure 1. Map of the study area. There are two small tributaries to the CLP above MWRF and below the MWRF: Dry Creek and Spring Creek. The information on flow of these two tributaries is fragmentary. Previous attempts at estimating the contribution of these two tributaries were not successful because of the absence of adequate hydrologic data. Because these tributaries have very low flow or no flow when CLP flows are lowest, estimation of DFLOW values of the CLP are not likely to be influenced by the tributaries. Because the observed low flow at the Mulberry WRF is known by observation to be very low at times of the year when flow in the river is lowest, errors that occur through lack of information on the small tributary flows will not be significant to the outcome of the DFLOW calculations. Also relevant is hydrology for the reservoir. Inflow of water to the reservoir and storage within the reservoir are recorded. The volume of water in the reservoir is relevant to potential for 3 dilution of effluent. The most extreme condition, however, occurs when the reservoir is dry, as shown in the water storage record. Therefore, for permitting analysis, dilution may be set to zero. It may also be necessary, however, to demonstrate what adverse effects, if any, could occur for entry of effluent when the reservoir is full or partly full. Exploration of this possibility will be included in the project. Constituent Concentrations Constituent concentrations for Mulberry WRF effluent and for water upstream of Mulberry WRF will be used in obtaining the necessary information on mixed flows involving upstream water and effluent from MWRF relevant to the new proposed CLP discharge point. The monitoring record is excellent. More difficult is assessment of water quality in the reservoir, given the absence of a monitoring record. For an extreme low storage value of zero, however, accurate estimation of water quality in the reservoir is not relevant. Possible adverse effects that occur when the reservoir contains water will be estimated. If data are made available by the City for use in the study, PEL estimates can be based in part on monitoring over a relatively short interval for this study. Regulatory Basis for Calculating Effluent Limits Calculation of PEL values will take into account the antidegradation policies that are applicable now to MWRF effluent. The antidegradation procedure requires that limits be computed for use protection purposes and separately for antidegradation (reviewable waters). Calculation of both of these types of limits is necessary because the State requires that the limits 4 be compared with each other, and that, for a given constituent, the more restrictive of the two limits be selected for setting the effluent limit or, in this case, the PEL. Benchmark water quality, as needed for antidegradation analysis, for the discharge to Timnath Reservoir will raise some questions. Typically benchmark water quality is determined by chemistry downstream of the discharge as determined from a monitoring record. The State allows, however, for other approaches. In this case, with potential for zero volume of lake water in Timnath Reservoir and zero ditch dilution, the most logical choice for a benchmark to be used in antidegradation analysis would be the recorded effluent chemistry for MWRF effluent over the last few years. This project will encompass all constituents that are relevant to MWRF permitting. Some constituents require customized calculations (Table 1). Constituent Basis for Calculation Ammonia Monthly effluent limits, with dependence on pH and temperature Temperature Weekly effluent limits (MWAT) Total P, Total N Annual limits based on medians Chlorophyll Indirect linkage to total P, total N Table 1. Constituents requiring special computational methods for PEL estimation. The scope of work includes three meetings at which presentation or discussion of project results will occur. Overview This product of this project will be a report that explains the basis for estimates of PEL values, describes the procedures by which the values were estimated, and shows the values in comparison with current effluent limits for MWRF. 5 Western Environmental Analysts 11446 County Road 23 Fort Lupton, CO 80621 December 7, 2016 Jason Graham, CWP Water Reclamation and Biosolids Manager City of Fort Collins The cost of the Mulberry WTF PEL project is $27,500, to be paid on a progress basis month by month with the final billing to occur after the report in final form has been sent to you. Work on the project will begin in February 2017. Sincerely yours, William Lewis President Western Environmental Analysts