APPENDIX B WATER QUALITY ASSESSMENT CD UD ------- CONTENTS Page LIST OF FIGURES iii LIST OF TABLES iv INTRODUCTION B-l B-I. SUSPENDED SOLIDS DEPOSITION B-2 SMALL DISCHARGER APPROACH B-2 LARGE DISCHARGER APPROACH B-6 B-II. DISSOLVED OXYGEN CONCENTRATION FOLLOWING INITIAL DILUTION B-15 B-III. FARFIELD DISSOLVED OXYGEN DEPRESSION B-23 SIMPLIFIED MATHEMATICAL MODELS B-25 NUMERICAL MODELS B-35 EVALUATION OF FIELD DATA B-36 B-IV. SEDIMENT OXYGEN DEMAND B-38 B-V. SUSPENDED SOLIDS CONCENTRATION FOLLOWING INITIAL DILUTION B-44 B-VI. EFFLUENT pH AFTER INITIAL DILUTION B-48 B-VII. LIGHT TRANSMITTANCE B-53 B-VIII. OTHER WATER QUALITY VARIABLES B-61 TOTAL DISSOLVED GASES B-61 CHLORINE RESIDUAL B-61 NUTRIENTS B-62 COLIFORM BACTERIA B-64 REFERENCES B-68 B-ii ------- FIGURES Number Page B-l Projected relationships between suspended solids mass emission, plume height-of-rise, sediment accumulation, and dissolved oxygen depression for open coastal areas B-3 B-2 Projected relationships between suspended solids mass emission, plume height-of-rise, sediment accumulation, and dissolved oxygen depression for semi-enclosed embayments and estuaries B-5 B-3 Example of predicted steady-state sediment accumulation around a marine outfall B-10 B-4 Dissolved oxygen deficit vs. travel time for a submerged wastefield B-28 B-5 Farfield dilution as a function of 12€0t/B2 B-33 B-iii ------- TABLES Number Page B-l Example tabulations of settleable organic component by group and maximum settling distance by group B-12 B-2 Example tabulations of deposition rates and accumulation rates by contour B-13 B-3 Typical IDOD values B-17 B-4 Dissolved oxygen saturation values B-21 B-5 Subsequent dilutions for various initial field widths and travel times B-41 B-6 Selected background suspended solids concentrations B-46 B-7 Calculated values for the critical effluent Secchi depth (cm) for selected ambient Secchi depths, initial dilutions, and a water quality standard for minimum Secchi disc visibility of 1 m B-58 B-iv ------- INTRODUCTION This appendix provides detailed guidance for responding to water quality-related questions in the Application Questionnaire. Methods for predicting values of the following water quality variables are presented: • Suspended solids deposition • Dissolved oxygen concentration following initial dilution • Farfield dissolved oxygen depression • Sediment oxygen demand • Suspended solids concentration following initial dilution • Effluent pH after initial dilution • Light transmittance • Other water quality variables. B-l ------- B-I. SUSPENDED SOLIDS DEPOSITION The applicant must predict the seabed accumulation due to the discharge of suspended solids into the receiving water. Two prediction methods are described in this appendix. The first is a simplified approach for small dischargers only. If this method is applicable, then a small discharger need not perform dissolved oxygen calculations dependent on settled effluent suspended solids accumulations. The second prediction method is applicable for both small and large dischargers. SMALL DISCHARGER APPROACH Two types of problems (dissolved oxygen depletion and biological effects) and two types of receiving water environments (open coastal and semi-enclosed bays or estuaries) are considered in the following approach. Figure B-l is to be used for open coastal areas that are generally considered well flushed. The dashed line represents combinations of solids mass emission rates and plume heights-of-rise that would result in a steady- state sediment accumulation of 50 g/m2. Review of data from several open coast discharges has indicated that biological effects are minimal when accumulation rates were estimated to be below this level. Consequently, if the applicant's mass emission rate and height-of-rise fall below this dashed line no further sediment accumulation analyses are needed. Applicants whose charge characteristics fall above the line should conduct a more detailed analysis of sediment accumulation discussed in the following section. The solid line in Figure B-l represents a combination of mass emission rates and plume heights-of-rise that were projected to result in sufficient sediment accumulation to cause a 0.2 mg/L oxygen depression. Applicants whose discharge falls below this solid line need not provide any further analysis of sediment accumulation as it relates to dissolved oxygen. B-2 ------- 7000 r- 6000 I » 5000 (ft 9 ui 4000 3000 2OOO 1000 I 024 6 6 1O 12 14 16 18 20 HEIGHT OF RISE, m STEADY STATE SEDIMENT ACCUMULATION LESS THAN 50g/m2 DO DEPRESSION DUE TO STEADY-STATE SEDIMENT DEMAND > 02 mg/l R«f«r»rx»: Tetra Tech (1982). Figure B-1. Projected relationships between suspended solid mass emission, plume height-of-rise, sediment accumulation, and dissolved oxygen depression for open coastal areas. B-3 ------- Figure B-2 should be used in a similar manner for discharges to semi- enclosed embayments or estuaries. Because estuaries and semi-enclosed embayments are potentially more sensitive than open coastal areas, the critical sediment accumulation was set at 25 g/m?. Methods described in Tetra Tech (1982) were used to determine the mass emission rates and heights-of-rise resulting in the sediment accumulation rates specified above. In order to use these methods, several assumptions were made. A current velocity of 5 cm/sec was assumed for the open coastal sites and a velocity of 2.5 cm/sec was assumed for the semi-enclosed embayments. these velocities are conservative estimates of average current velocities over a 1-yr period. The settling velocity (Vs) distribution used is considered typical of primary or advanced primary effluents and is shown below: 5 percent have Vs > 0.1 cm/sec 20 percent have Vs > 0.01 cm/sec 30 percent have Vs > 0.006 cm/sec 50 percent have Vs > 0.001 cm/sec The remaining solids settle so slowly that they are assumed to remain suspended in the water column indefinitely. The effluent is considered to be 80 percent organic and 20 percent inorganic. The above distribution is based on the review of data in Section 301 (h) applications and other published data (Myers 1974; Herring and Abati 1978). The annual suspended solids mass emission rate should be calculated using the average flow rate and an average suspended solids concentration. The plume height-of-rise, determined previously in the initial dilution calculation, or 0.6 times the water depth, whichever is larger, should be used to enter the appropriate figure (Figure B-l or B-2). B-4 ------- 4000 r- 0 3000 o (A o uj 2000 cc I M 1000 (A 55 6 8 10 12 U 16 HEIGHT OF RISE, m 20 STEADY STATE SEDIMENT ACCUMULATION LESS THAN 250/m2 DO DEPRESSION DUE TO STEADY-STATE SEDIMENT DEMAND > 0.2 mg/l Reference: Tetra Tech (1982). Figure B-2. Projected relationships between solid mass emission, plume height-of-rise, sediment accumulation, and dissolved oxygen depression for semi-enclosed embayments and estuaries. B-5 ------- LARGE DISCHARGER APPROACH The approach described here considers the processes of sediment deposition, decay of organic materials, and resuspension. However, the strictly quantitative prediction of seabed accumulation is based only on the processes of deposition and decay. Because resuspension is not evaluated easily using simplified approaches, the analyses described in this chapter consider resuspension separately and in a more qualitative manner that is based on measured near-bottom current speeds in the vicinity of the diffuser. Data Requirements To predict seabed deposition rates of suspended solids, the following information is required: • Suspended solids mass emission rate • Current speed and direction • Height-of-rise of the plume • Suspended solids settling velocity distribution. The mass emission rate, M (kg/day), is: M = 86.4(S)(Q) B-l where: S » Suspended solids concentration, mg/L Q - Volumetric flo* rate, m3/sec. It is suggested that the applicant develop estimates of the suspended solids mass emission rate for the season (50-day period) critical for seabed B-6 ------- deposition and for a yearly period. If the applicant anticipates the mass emission rate will increase during the permit term, the mass emission rate at the end of the permit term should be used. Current-speed data are needed to determine the distance from the outfall that the sediments will travel before accumulating on the bottom. Consequently, depth-averaged values are best, if available. Otherwise, current speeds near mid-depth may be sufficient. The following current data are needed for the assessment: • Average value upcoast, when the current is upcoast • Average value downcoast, when the current is downcoast • Average value onshore, when the current is onshore • Average value offshore, when the current is offshore. If no current data are available, values of 5 cm/sec for longshore transport and 3 cm/sec for onshore-offshore transport have been found to be reasonable values. Plume trapping levels representative of the critical 90-day period and of the annual cycle are needed. The applicant should use density profiles, effluent volumetric flow rates, and ambient currents characteristic of these time periods. Extreme values should not be used. Usually the annual average and critical 90-day average flow rates and current speeds (in the predominant current direction) should be used. The expected average plume heights-of-rise above the seafloor should be determined using available receiving water density profiles. If large numbers of profiles exist for each month (or oceanographic season), then the applicant could compute the plume height-of-rise above the seafloor for each of the available profiles, and then average the heights. If relatively few profiles are available for each month, then the applicant could compute the plume height of risk for each profile and use the lowest height-of-rise as the average. The monthly B-7 ------- average heights of rise can then be used to compute the average height-of- rise for annual and critical 90-day periods. If so few profiles exist that it is not possible to determine whether differences exist between months (or oceanographic seasons), then the applicant should use the lowest plume height-of-rise (based on calculations using the average effluent flow and current speed) as the average height-of-rise for both the annual and critical 90-day periods. If the applicant has not determined a suspended solids settling velocity distribution, the following can be used based on data from other Section 301(h) applications: Primary or Advanced Primary Effluent Raw Sewage 5 percent have Vs >0.1 cm/sec 5 percent have Vs >1.0 cm/sec 20 percent have Vs >0.01 cm/sec 20 percent have Vs >0.5 cm/sec 30 percent have Vs >0.006 cm/sec 40 percent have Vs >0.1 cm/sec 50 percent have Vs >0.001 cm/sec 60 percent have Vs >0.01 cm/sec 85 percent have Vs >0.001 cm/sec. The remaining solids settle so slowly that they are assumed to remain suspended in the water column indefinitely (i.e., they act as colloids). Consequently, 50 percent of the suspended solids in a treated effluent and 85 percent of those in a raw sewage discharge are assumed to be settleable in the ambient environment. Prediction of Deposition Although a portion of the settled solids is inert, primary concern is with the organic fraction of the settled solids. For purposes of this evaluation, composition of the waste discharge can be assumed to be as fol1ows: B-8 ------- • 80 percent organic and 20 percent Inorganic, for primary or advanced primary effluent • 50 percent organic and 50 percent inorganic, for raw sewage. Accumulation should be predicted for the critical 90-day period when seabed deposition is likely to be highest and for steady-state conditions where average annual values are used. The results should be presented in graphical form, as shown in Figure B-3. Supporting tables should be submitted with the application. The applicant must exercise judgment when developing the contours, especially when accounting for rapid depth changes offshore. Sediment contours should be expressed in units of g/m?, not as an accumulation depth. An applicant may use a proprietary or publicly available sedimentation model. Two widely known models are those of Hendricks (1987), which has been used extensively offshore of Palos Verdes Peninsula in the Southern California Bight, and Farley (Tetra Tech 1987), which describes the Ocean Data Evaluation System (ODES) model DECAL. The model DECAL is publicly available through the U.S. EPA. A simple model is described herein. It can be used to obtain acceptable estimates of sediment accumulation in a variety of environments. If its use results in sediment accumulations that lead to violations of state standards or federal criteria for receiving water quality, an applicant may use a more sophisticated effluent sediment accumulation model that better simulates the marine environment. The method described below assumes that effluent sediment particles having a specific particle fall velocity settle uniformly within an elliptical area. This area depends on the plume height-of-rise relative to the seafloor (not the port depth), the particle fall velocity, and the average currents speeds in four directions (upcoast, downcoast, onshore, and1 offshore) appropriate for an effluent wastefield at the plume height-of-rise. For the following sample calculations, the diffuser was assumed to be a point source. Use of this assumption may not produce reasonable estimates of sediment accumulation if the diffuser is long. If the diffuser is long, B-9 ------- 180 o i I i l nautical mites I I i kilometers o 2 CONTOURS IN FEET Figure B-3. Examples of predicted steady-state sediment accumulation around a marine outfall. B-10 ------- then estimates of the sediment accumulation from each diffuser port can be summed to obtain an estimate for the entire diffuser. This sum is approxi- mately the same as that obtained from assuming that the sediment accumulation area is a ZID-like area (with ends the same as the similar elliptical halves computed for a single point discharge) and that the effluent suspended solids having the specific particle fall velocity uniformly settle in this area. The sediment accumulation due to the entire discharge is the sum of the accumulations for each particle fall velocity modeled. To begin computations for a discharge at a point location, the applicant can create a table similar to Table B-l, showing the amount of organic solids that settle within each settling velocity group, and the maximum distance from the outfall at which each group settles. If the applicant has current data for more than four quadrants, those data can be used. The maximum settling distances for each group in each direction are calculated using the formula shown in the footnote of Table B-l. With a sufficiently detailed map (e.g., a NOAA bathymetric chart), each point DI through 0^5, or Rj through R2Q> can be plotted with the center of the diffuser as the reference point. Depositional contours are formed by the four points that define the perimeter of a depositional field (e.g., 0^0304). The applicant should join these points by smooth lines, so that the contours are elliptically shaped. If the applicant has current data at 60° or 30° intervals, instead of the 90° intervals used here, then the contours could be created more accurately. The deposition rates corresponding to each contour are determined as follows. First, predict the deposition rate within each contour due to each individual settling velocity group, as shown in Table B-2. This quantity is Mi/A-j, or the group deposition rate divided by the area within the contour. The area within any contour is a function of the four points (e.g., DJ, 02, 03, and 04), and is denoted in the table by f(0^20304). A planimeter is probably the most accurate method of finding the area. Once the deposition rates by group have been found, then the total deposition rate can be calculated by summing all contributing deposition rates. For B-ll ------- TABLE B-1. EXAMPLE TABULATIONS OF SETTLEABLE ORGANIC COMPONENT BY GROUP, AND MAXIMUM SETTLING DISTANCE BY GROUP Mass Emission Rate • MT Organic Component * Mo » Percent by Settling 0.8 My, for primary effluent 0.5 MJ., for raw effluent Organic Component Maximun Settling Distance from Outfall* Velocity Group Primary Effluent 5 (V, > 0.1 cm/sec) 15 (V * 0.01 cm/sec) 10 (V, a 0.006 cm/sec) 20 (V « 0.001 cm/sec) by Group 0.04 MT 0.12 M} 0.08 MT 0.16 Hj Upcoast o1 °5 D13 Oowncoast D6 D14 Onshore Offshore D15 °16 Sum » 0.40 MT Ran Sewage 10 10 20 20 25 (Vs s |