Using the Waste Load Allocation Model (WLAM) to Develop Effluent Limitations and Waste Discharge Requirements

Transcription

1 Using the Waste Load Allocation Model (WLAM) to Develop Effluent Limitations and Waste Discharge Requirements Background The Regional Water Quality Control Board establishes water quality objectives to protect the designated beneficial uses of surface and ground waters in the Santa Ana River watershed. These objectives are set forth in the Regional Water Quality Control Plan (aka "Basin Plan"). And, the Regional Board imposes effluent limitations or waste discharge requirements (WDRs) to ensure compliance with the water quality objectives. When issuing discharge permits, the Regional Board must take into consideration a wide range of factors in order to determine the most appropriate effluent limitation. This includes, but is not limited to: the water quality objectives, the current average water quality in the receiving water, the availability (or lack thereof) of assimilative capacity in the receiving water, the net effect of the regulated discharge on the receiving water (alone and in combination with all other discharges to the same receiving water), the volume and quality of other natural and man-made flows reaching the receiving water, etc. Theoretically, the Regional Board could simplify the permitting process by establishing effluent limitations that prohibited pollutant concentrations from exceeding the applicable water quality objective or the current average concentration in the receiving water (whichever was more restrictive). However, such an approach, while administratively easier, would not be consistent with wise water resource management. And, in particular, would tend to discourage greater use of recycled water in the region. Working closely with stakeholders throughout the watershed, the Regional Board developed a more sophisticated tool for deriving appropriate effluent limitations and waste discharge that will ensure long-term compliance with the nitrogen and salinity objectives in the Basin Plan. This tool, called the Waste Load Allocation Model (WLAM), evaluates the cumulative effects of a large number of different discharges, over a wide range of land use and planning conditions, on the surface and ground waters of the region. The WLAM also takes into consideration the normal fluctuations in weather patterns (e.g. extend droughts or El Niño winters) that also influence regional water quality. The WLAM is a powerful tool designed to aid permit writers in the process of deriving appropriate effluent limitations and waste discharge requirements. This document is intended to describe how the published WLAM results should be interpreted and used to achieve this purpose. Oct. 24, 2012 DRAFT: For Discussion Purposes Only Page 1

2 What Is the WLAM? In simple terms, the Waste Load Allocation Model is spreadsheet tool designed to predict the concentration of Total Inorganic Nitrogen (TIN) and Total Dissolved Solids (TDS) that will recharge to ground water through streambed percolation in the Santa Ana River and each of its major tributaries. The WLAM estimates the volume and quality of ground water recharge by summing the volume and quality of all known sources to any given stream segment (incl. inflows from upstream tributaries, storm water runoff, waste water discharges and rising ground water). The WLAM uses historical weather data for a 50-year period and current and projected land use to estimate the volume of runoff likely to reach each stream segment. The WLAM also considers the volume and quality of waste water discharges to local surface streams. And, the WLAM evaluates several different recycled water planning scenarios to estimate how the volume and quality of waste water discharges is likely to change over the next 10 years. A more detailed explanation of precisely how the calculations are performed is provided in the WLAM Final Report. The WLAM Final Report is a large document filled with detailed tables and graphs that describe the estimated recharge quality expected to occur at several key locations, under a dozen different planning assumptions, over a typical 50-year period of changing precipitation. Consequently, the sheer volume of results in the can be somewhat daunting to those using the WLAM Final Report for the first time. This memorandum is intended ease that introduction. What are the WLAM's Limitations? The WLAM is designed to estimate the quality of water that will percolate to local ground water basins from surface stream overlying those aquifers. It is not intended to quantify precisely how the average TIN or TDS concentrations in the ground water will change in response to these surface water recharges. Similarly, the WLAM is not designed to determine the rate at which assimilative capacity will increase or diminish over time. The WLAM does not take into consideration any other sources of ground water recharge that occurs in areas outside the stream channels. So, for example, ground water recharge from agricultural or landscape irrigation is not included in the WLAM. Nor is precipitation that percolates "on-site" included in the WLAM calculations. The WLAM is not designed to determine the maximum amount of TIN or TDS that can be discharged at any given location without exceeding the applicable water quality objectives. Rather, the WLAM is used to determine whether the current and planned volume and quality of waste water discharges will comply with the Basin Plan objectives. It is designed to make this determination while taking into consideration probable future land uses, a variety of different weather conditions, and a wide range of water reclamation scenarios. Oct. 24, 2012 DRAFT: For Discussion Purposes Only Page 2

3 As long as the volume and quality of TIN and TDS in a proposed waste water discharge does not exceed the range of parameters evaluated within the WLAM, permit writers can rely on the WLAM results to derive an effluent limit that will be expected to achieve the applicable water quality objectives. However, if the average concentration of TIN or TDS, or the total volume of effluent, is greater than previously evaluated in the WLAM, it may be necessary to re-run the model using the higher values to assure continued compliance with the objectives. The WLAM has only been calibrated to estimate the volume of stream flow and concentration of TIN and TDS in the streams. It has not yet been calibrated for any other water quality parameters including the individual ions that, collectively, comprise TDS. Consequently, it should not be used to derive effluent limitations for any other chemical pollutants that may be present in the reclaimed water discharged to these same streams. The WLAM necessarily makes a series of conservative assumptions. For example, the WLAM assumes that all of the wastewater treatment plants are operating at their full permitted capacity. The WLAM further assumes that TIN and TDS is being discharged at the maximum permitted effluent concentration. It is virtually impossible for these conditions to exist simultaneously throughout the watershed. Nevertheless, the resulting "worst case" analysis makes it easier to authorize permits without requiring more expensive and time-consuming studies unless absolutely necessary. Results for the WLAM are limited to the planning assumptions used to run the model. It is presumed that the WLAM will be updated on a regular basis to ensure that the planning assumption continue to represent a range of reasonable expectations. For example, the current WLAM provides water quality estimates for land use conditions expected to occur between 2010 and 2020 (a 10-year planning horizon). Sometime prior to 2020, the WLAM should be updated to reflect land use conditions that are likely to occur in the period between 2020 and Finally, the WLAM is not intended to be the "final word" on whether a given discharge can be permitted or the exact nature of the effluent limitations imposed within that permit. Rather, it is intended to provide a general tool for assessing whether a specific discharge is likely to comply with the applicable water quality objectives given a large number of other relevant factors that must be considered. The WLAM results are meant to inform, not make, the decision. The significance of this caveat will be apparent in some examples to follow. Oct. 24, 2012 DRAFT: For Discussion Purposes Only Page 3

4 Interpreting the WLAM Results The WLAM calculates stream flow and water quality on a daily time step. Results of this analysis are summarized into average values representing 1-year, 10-year and 50-year time frames. The annual average is intended to provide some indication of how dramatically water quality can vary from year to year. But, the 50-year average better describes the expected long-term effect of surface water recharges to the underlying ground water basin. The ground water objectives identified in the Basin Plan were established by using the best available data to calculate the volume-weighted average concentration of TIN and TDS in each ground water basin (aka "management zone") over a 20-year period. Every three years, the average TIN and TDS concentration is re-estimated using the most recent 20 years of data collected from wells throughout the region. The 20-year period was originally selected to ensure there would be sufficient data to provide a robust estimate of average ground water quality and to ensure that the such estimates accurately accounted for the wide range of meteorological conditions that affect recharge from the surface. The 10-year estimate of average recharge quality was recommended as a first-order threshold value for assessing whether a given waste load allocation scenario would comply with the Basin Plan objectives. The 10-year estimate is relatively conservative because the time frame is half of that used to set the water quality objectives themselves. And, as such, provides a reasonable early warning indication of waste discharge scenarios that may eventually lead to exceedances of the water quality standards. It is important to note that the WLAM provides an annual estimate of water quality for each of 50 different years. Then, it uses the annual estimates to calculate a 10-year running average. The 10-year running average changes from year to year as old data falls out of the calculation and new data is added. Consequently, the WLAM provides 50-different, estimates of the 10- year running average for each of the unique land use and planning scenarios. For convenience, the results are frequently summarized in a large table like that shown in Figure 1. It is important to note that the table in Figure 1 only reports the highest 10-year average observed in a series of fifty different 10-year averages. A more detailed table showing shows all of the single year values as well as the 10-year running average for each year for that portion of the Santa Ana River overlying the Riverside-A ground water basin is shown in Figure 2. Even then, the vast amount of data makes it difficult to understand and interpret all the information. This is best accomplished using a graphic format. Figure 3 illustrates how TIN concentrations vary in the stream water that percolates to the Riverside-A ground water basin for just one of the planning scenarios. This example assumes wastewater agencies are meeting 100% of their goal for recycled water reuse under 2020 land use conditions. Oct. 24, 2012 DRAFT: For Discussion Purposes Only Page 4

5 The black line in Figure 3 accurately reflects how the annual average TIN concentrations varies within a range between 3 mg/l and 7 mg/l. The red line represents the 10-year running volume-weighted average. It, too, varies but within a much narrower range (approximately mg/l). And, the blue line signifies the volume-weighted average of all 50 years. In this case, the average concentration of TIN percolating to the Riverside-A ground water basin from Reach 4 of the Santa Ana River is approximately 5.5 mg/l. Although the long-term average is less than the TIN objective of 6.2 mg/l assigned to the Riverside-A management zone (indicated by the yellow line), the 10 year running average occasionally exceeds the Basin Plan objective. For example, in Figure 3, it appears that the objective might be exceeded if a prolonged drought like that which occurred in the early 1960's were to recur. Referring back to the table in Figure 2, we see that the 10-year running average reached a peak value of 6.24 mg/l under such conditions. Charts like the one depicted in Figure 3 can also be used to determine whether the sum of all discharges in a given reach are likely to degrade water quality in the underlying ground water basin. In this example, the volume-weighted average TIN concentration in the Riverside-A management zone is 4.9 mg/l (note: graph must be revised to add a line indicating the current ambient quality). Because the assigned objective is 6.2 mg/l, there is 1.3 mg/l of assimilative capacity available. And, since the average TIN concentration in water percolating from Reach-4 of the Santa Ana River to the Riverside-A basin is 5.5 mg/l, it is possible that the recharge will begin to degrade the water quality over time. The question of how much and how quickly the available assimilative capacity may diminish is not one that WLAM was designed to address. The answer depends on the volume and location of streambed percolation and the volume and quality of other sources of recharge to the same ground water management zone. Using the WLAM to Make Permitting Decisions By itself, the fact that the WLAM predicts TIN levels in the recharge water will occasionally exceed the objective does not preclude the Regional Board from authorizing discharge permits consistent with the underlying assumptions in this scenario. Recall that the WLAM only calculates the quality of water as it percolates to ground water, it does not estimate what effect (if any) this recharge will have on the underlying ground water. In this example, there is no reason to conclude that average TIN concentration in the Riverside-A management zone will violate the objective just because the stream water percolating to that zone sometimes exceeds the objective. Oct. 24, 2012 DRAFT: For Discussion Purposes Only Page 5

6 As noted earlier, the WLAM is designed to facilitate the regulatory analysis not serve as a substitute for it. The 10-year running average is provides a conservative threshold intended to trigger more detailed analysis where warranted. For the case shown in Figure 3, it is evident that the frequency, duration and magnitude of the exceedance are all small. The event only occurred once in 50 different estimates of the 10-year running average. This suggests a 2% probability of recurrence. The duration of the exceedance was temporally-limited - lasting for only a single year. And, the magnitude of the exceedance is less than one-half of 1% over the objective (i.e mg/l vs mg/l, respectively). Consequently, it is reasonable to conclude that wastewater discharges that are permitted in accordance with the assumptions underlying this particular planning scenario are unlikely to cause or contribute to an exceedance of the TIN objective in the Riverside-A management zone. In this example, however, the concerns regarding the potential for water quality degradation are likely to be more significant. As noted earlier, the current volume-weighted average TIN concentration in the Riverside-A management zone is approximately 4.9 mg/l and is well under the Basin Plan objective of 6.2 mg/l. All of the projected 10-year running averages are greater than 4.9 mg/l and the long-term (50-year) average TIN concentration in the streambed recharge is 5.5 mg/l. Therefore, it is reasonable to conclude that some degradation is likely to occur over time. In accordance with the state's Recycled Water Policy, the Regional Board can and should authorize discharge permits consistent with the example scenario provided that the sum of all such discharges recharging to ground water consume less than 20% of the available assimilative capacity. Additional mass-balance analysis may be needed to determine precisely if and how the existing assimilative capacity will be affected. Oct. 24, 2012 DRAFT: For Discussion Purposes Only Page 6

7 Table 4 Summary of TIN Wasteload Allocation Model Results for Scenario 7 Reach Underlying Management Zone Nitrate- Nitrogen Objective Current 1 Ambient NO 3 -N Assimilative Capacity (mg/l) (mg/l) (mg/l) Compliance Metric Scenario 7a Scenario 7b Scenario 7c Scenario 7d Scenario 7e Scenario 7f Planned Reuse in 2010 Model Results for TIN (mg/l-n) Partial Reuse in 2010 Maximum Discharge in 2010 Planned Reuse in 2020 Partial Reuse in 2020 Maximum Discharge in 2020 Surface Water Santa Ana River Reach 3 at Below Prado Dam na 10* 4.3* Maximum of flow-weighted average TIN na** in August Santa Ana River Reach 2 at Below Prado Dam na na na na** Maximum of 5-year moving average of annual flow-weighted average TIN Groundwater Santa Ana River overlying the Chino-South MZ Chino-South none Santa Ana River overlying the Riverside-A MZ Riverside-A Santa Ana River overlying the Colton MZ Colton none Santa Ana River from the San Jacinto Fault to confluence with San Timoteo Creek; San Timoteo Creek overlying the Bunker Hill-B MZ Bunker Hill-B San Timoteo Creek in the San Timoteo MZ; Cooper's Creek in the San Timoteo MZ San Timoteo 5 --*** --*** San Timoteo Creek in the Beaumont MZ; Noble Creek below Beaumont DP 008; Unnamed tributary to Marshall Creek below Beaumont DP 007 Beaumont Notes * August only; Total nitrogen (filtered samples) ** Currently, the Regional Board does not recognize the existence of assimilative capacity for nitrogen in the Santa Ana River *** Insufficient data to determine current ambient TIN and, therefore, assimilative capacity (Wildermuth, 2008). 1 Current ambient represents 2008 conditions for surface water and conditions for groundwater Table_3_4.xls -- Table_4_TIN 7/20/ Draft Version 1

8 Table 7d-RA TDS and TIN of Streambed Recharge to the Riverside-A Management Zone Scenario 7d - Planned Reuse in 2020 Volume-Weighted Running Average (mg/l) Water Year TDS TIN 1 Year 10 Year 50 Year 1 Year 10 Year 50 Year Maximum R_S7d1_matrix.xls -- Table2 1/15/2010

9 10 Annual Recharge to Riverside A MZ Figure 7d-TIN_RA Estimated Annual Streambed Recharge and Volume-Weighted TIN Concentration of the Santa Ana River to the Riverside-A Management Zone Scenario 7d - Planned Reuse in 2020 Nitrate-N Objective for Riverside A MZ (6.2 mg/l) 1-Year Volume-Weighted Average for TIN in Recharge 10-Year Volume-Weighted Running Average for TIN in Recharge 50-Year Volume-Weighted Average for TIN in Recharge (5.52 mg/l-n) 50, ,000 TIN (mg/l-n) ,000 20,000 Streambed Recharge (acre-ft/yr) 2 10, Water Year 0 R_S7d1_matrix.xls -- Fig 2b

10 If Assimilative Capacity Exists in the Ground Water Management Zone Is Effluent Quality < Current Ambient Quality? Is Recharge Quality < Current Ambient Quality in ALL projected 10-year running averages? Is the Long-term (50-year) Recharge Quality < Current Ambient Quality? Will the sum of all recycled water recharging the management zone reduce the assimilative capacity by more than 20%? Will the recycled water recharging the management zone from this one permitted discharge reduce the assimilative capacity by more than 10%? Will the receiving water in the management zone continue to meet the water quality objective? Will permitting additional water quality degradation provide "maximum benefit to the people of California?" Issue permit using discharge flows and TIN/TDS levels assumed in the WLAM. Issue permit using discharge flows and TIN/TDS levels assumed in the WLAM. Do not permit additional water quality degradation.