Environmental Cleanup & Restoration Design Project Summer 2002 Overview

Transcription

1 Environmental Cleanup & Restoration Design Project Summer 2002 Overview Natrium, Oregon is a coastal city known for its waterfront park and summer Saltwater Taffy festival held on the banks of the Nackel River, a broad and well-mixed saline estuary. The Natrium Chamber of Commerce has worked hard to develop the Taffy Waterfront Park into a major tourist attraction, and has recently added a new parking lot and shops. Public showers have been added at the park using the local groundwater. Routine water quality sampling showed elevated levels (1.7 ppb) of trichloroethylene (TCE) in the groundwater. Your company has been contracted to determine the plume characteristics and the ensuing remediation strategy. The Oregon Department of Environmental Quality (DEQ) has recommended that remedial measures be under construction, if not operational, within a year. The city (the owner) would like to begin remediation as soon as possible. Neither the city nor DEQ wants to let the plume reach the nearby Nackel River estuary (if it s not there already). Your firm submitted a successful (lowest cost) bid that includes a remedial investigation (RI) budget for the initial year of $35,000. The goal of the RI is to gather data to determine plume size (extent), strength, and movement. Summary of Hydrogeology: The site is located in the built-up central portion of Natrium, OR, a small community bordered to the west by the estuarine Nackel River (average salinity of 10 parts per thousand) and to the north and east by a small perennial stream (see Figure 1). The Nackel River is broad and relatively free of tidal effects. The river can be assumed to have a constant stage, or water level elevation. The contaminants are in a shallow surface aquifer characterized as a silty-sand to sand with intermittent silt and clayey-silt lenses. The aquifer is 25 to 35 meters thick and thickest midway between the highway and the Nackel River (see Figure 2.) The saturated thickness is closer to 20 meters over most of the site. Beneath this shallow aquifer is a layer of silt and clayey-silt which acts as an effective aquitard. The groundwater flow is expected to generally flow towards the river. The City of Natrium receives an average of 60 inches per year of rainfall, distributed roughly evenly over the year. One-third of the precipitation is expected to reach the surface aquifer and comprises the bulk of the shallow aquifer recharge. A perennial stream located about 350 m northeast of the site and contributes to the local water table. A hydrogeologic summary follows, as estimated by county survey maps: Table 1: Hydrogeologic characteristic summary Characteristic Value range Estimated value Average aquifer thickness ft 60 ft Transmissivity 350 to 1000 ft 2 /day 600 ft 2 /day Hydraulic conductivity 7 to 15 ft/day 10 ft/day Porosity 0.25 to Specific Yield 0.8 to

2 The only known well within 500 meters is operated by the Silver Saw Company, located about 220 meters to the east. It is reported to pump at an average rate of 8 gpm. Initial predictions by the firms chemical staff estimate that retardation of contaminants should be negligible. The TCE is expected to move as a conservative tracer, (i.e., move at the same speed at the groundwater). Project The project is broken up into two independent phases: Phase I: RI: Investigation and characterization of the plume Goals: 1) Establish the present location and severity of the VOC plume focusing on TCE as the primary contaminant of concern (COC). 2) Predict the future movement of the plume if no action is taken. Outcomes (specific tasks you are to accomplish): 1) Determine hydraulic gradient from piezometer data. 2) Determine plume speed using Darcy s law. 3) Draw an approximate map of the VOC plume and the concentration within the plume. 4) Utilize Darcy s Law and the flow field to forecast future plume movement. Report: Submit a report summarizing your findings for the specific tasks listed previously. Provide a breakdown of the investigation budget expenditures (anything left over is profit for you), a contour map of the plume concentration, and a prediction of the plume movement if no action is taken. Assume the investigation starts in August, Your report will include: 1) A map showing the current location of the plume, the location of the piezometers and wells, and an estimated contour map of the plume. [This may be done neatly by hand. It must be drawn to scale.] Note that the values of the contour lines need not be evenly spaces: you could use 1, 10, 50, and 100 ppb for example. 2) Report the estimated volume of contaminated water in the saturated portion of the aquifer. [First approximation only, say ± 25%.] Briefly discuss the accuracy of this estimate. 3) Report the local and regional gradients. [no plot is required] 4) Report the probable plume seepage velocity (the speed of the center of mass of the plume). 5) Discuss how the plume will behave if no action is taken. 2

3 Data Acquisition: To make the exercise realistic, you will obtain data the way a consultant does by requesting it from a subcontractor who collects field data and bills you for the cost. To simulate this process you will get field data by submitting requests. When submitting monthly requests, include Team/Project Number, month and year, Action Code, the installation locations (x and y coordinates) for any new piezometers or wells, and the well name designations (for future use in subsequent requests). Requests will by made by ing the TA at mckillim@cecs.pdx.edu. Your boss has already acquired a survey map of the area with the location of the city well and the Silver Saw well included. Unfortunately, this cost $500. This site map will be used to determine the coordinates of your piezometers and monitoring wells. To use this map, load the supplied IGW model and discretize it. The tip of the arrow cursor can then be used to determine the x-, y-coordinates. Action Action Code Unit cost Basemap (survey) of the site BM $500 Installation of a piezometer (yields IP-(x-location, $700 head data only) y-location, name) Installation of a monitoring well (yields IM-(x-location, $2000 head and concentration data) y-location, name) Monthly reading of head data RH-(names) $100 Monthly reading of head and sampling RC-(names) for laboratory determination of $300 concentration Sample and test a piezometer for contaminant detection. (poor accuracy) TP-(names) $300 Example: Team 1, for their first month is investigating, the water table elevations at three locations. They request the installation and reading of three piezometers, named P-1, P-2, and P-3, and identified by two map-grid coordinates of their IGW model. The lower left corner of the model is the coordinate system origin (i.e. x=0, y=0). The request to the TA looks like this: Team 1. August, BM IP-(200 m, 300 m, P-1). IP-(200 m, 200 m, P-2). IP-(150 m, 250 m, P-3). RH-(P-1, P-2, P-3). The reply receipt looks like this: Team 1. August, Starting RI balance $35,000. BM $500 IP-(200 m, 300 m, P-1). $700 IP-(200 m, 200 m, P-2). $700 IP-(150 m, 250 m, P-3). $700 RH-(P-1=2.90 m, P-2=2.84 m, P-3=2.20 m). $300 Ending RI balance $32,100. 3

4 For simplicity, actions will occur at monthly intervals. I.e., the data collection will occur once a month. You have one virtual year to complete your RI (August 2002 to July 2003). Thus, you have twelve sampling periods. N Figure 1: Site map for the VOC detection, Natrium, OR. (2002) This map is 480 meters wide. Figure 2: Cross section AA. Horizontal datum is the left edge of Figure 1. 4

5 Phase II: Feasibility Study: Planning the clean-up Once you have established the extent, severity, and probable behavior of the plume contaminating the public showers well, you must develop a feasible remediation strategy. Assume the contamination source has already been removed. After you have completed Phase I, the TA to obtain the IGW model for Phase II. Goals: 1) Select among natural attenuation, pump and treat, and pump and treat with hydraulic control as the remediation strategy to be pursued. 2) Design the selected remediation strategy. Remediation design: Utilize the IGW (Interactive Groundwater) software and model provided to evaluate treatment strategies, (through trial and error at worst). At this point, the plume size and strength is completely known. The groundwater modeling has already been done, so the focus will be on the treatment, not the diagnostics or modeling. A tutorial session for the software will be provided, if needed, and help will be available. Outcomes: 1) Recommend and justify your selection for the remediation strategy. 2) Design the recommended remediation strategy. Specifically, subject to the constraints below, report: a. well locations (by coordinates) and individual pumping rates b. maximum treatment rate (same as the sum of all pumping rates of any water to be treated). [This rate would be used to site the above-ground treatment units.] c. total treated and untreated (if applicable) pumping rates. [This rate is the amount of water which must be discharged from the site, q.v. design constraint #3.] d. estimated time (in years) to reach the interim remediation goal e. estimated number of pores volumes of water pumped to reach the interim remediation goal Report: Submit a report that summarizes your results for the above tasks. Be sure to include a map showing the location of any wells. IGW screen captures can be used for this map. Also include a couple screen captures showing the predicted plume response to remediation. (E.g. one initial, one intermediate, one at completion.) The IGW tutorial gives information on making screen captures. 5

6 Design Constraints: 1) The treatment goal is the EPA MCL for TCE, which is 5 ppb. 2) Protection of the groundwater is important; no saltwater intrusion is allowed (i.e., do not induce flow from the river toward the aquifer). 3) The waste-water facility cannot reliably accommodate additional inflows in excess of 0.25 MGD which limits the total discharge of treated and untreated water. 4) Wells may not be placed in buildings, roads, or the main parking lot. I.e., the colored portions of the basemap. (Sometimes IGW will force a well into a colored portion when it moves the wells to the grid nodes. We can work around this.) 5) The pumping well discharge may be considered either treated or untreated: a) Treated inflow may be of any concentration (it s to be treated!) b) Untreated inflow must be below the detection limit (i.e., zero in the IGW model) 6) The design should include some flexibility. Consider the following: (Note: an exhaustive consideration is outside the scope of work) a) Aquifer parameters are estimates and averages: real systems are heterogeneous. b) Hydraulic gradients (and flow fields) may change directions and magnitudes due to natural or anthropogenic causes. c) The picture of the plume is never complete or omniscient: it may have undiscovered fingers or concentration spikes. d) Pumping wells typically utilize submerged single speed pumps: Changing pumping rates entails considerable cost. In comparison, pumps are easily turned on and off. Design Assumptions & Simplifications: 1) The TCE plume is assumed to be well-mixed vertically in the aquifer. 2) The source of the plume and the bulk of the contaminated soil have been removed. Assume no further source (input) of contaminants. 3) Well and piezometer depths are appropriate for the determination of hydraulic head and the sampling of the plume concentration. 4) Cost considerations for the remediation design are outside the scope of this project. 5) While no time limit to reach the remediation goal exists in this project, the client would like to be finished sooner than later, and sometime rather than never. I.e., faster remediation is preferable. 6) The river stage is uniform and steady, (i.e., constant.) 7) The recharge rate is uniform and steady, (i.e., constant.) 8) Assume retardation and chemical decay are negligible. 9) Assume the numerical dispersion introduced by IGW is comparable to the actual dispersion. 6

7 Grading Phase I: An A project will have all of the following: 1) An accurate image of the hydraulic gradient in the vicinity of the plume, particularly in the critical region between the source, the well, and the lake. 2) An accurate determination of the mean plume velocity (i.e., seepage velocity in the area of concern.) 3) A reasonably accurate estimate (± 25%) of the volume of contaminated water in the aquifer. 4) An accurate forecast of what the plume will look like in future years. For example, what would the plume look like after it passed the well and reached the lake. 5) Any sampling scheme that seems logical and is consistent with the conclusions you draw. No points for lucky guesses. 6) A budget that falls well within the $35,000 constraint. A B project is deficient in one or two of the above areas. A C project is deficient in three areas or is seriously deficient in at least two of them. A D project is four or more areas or is seriously deficient in at least three of them. Failure is not an option! Phase II: An A project will have all of the following: 1) A remediation approach that clearly meets the requirements of the cleanup with respect to TCE concentrations. 2) A succinct and clean justification for this remediation strategy. 3) Remedial measures plan that is logical and well thought out. For example, the well arrangement is logical rather just a semi-random scattering of wells. 4) A complete and easy to understand reporting of all design characteristics and parameters. 5) A realistic estimate of the time to reach the remediation goal, verified by simulated results with IGW. A B, C, or D are as per the grading for Phase I. Addenda: The IGW software (use version 3.3) and tutorial are available at 7