Contaminant Hydrogeology Organic Partitioning and Reactive Transport Processes - Engineering Assignment Help

Hydrogeology Engineering Assignment Answer

Objectives: • Evaluate how organic contaminants near a source are partitioned between various phases in the subsurface • Continue your site investigation by considering how reactive contaminants are transported in groundwater • Examine the importance of sorption and biodegradation as mechanisms for attenuating plume migration • Become familiar with the program BIOSCREEN-AT as a tool for evaluating contaminant transport and biodegradation processes. Introduction and Problem Description You are asked to evaluate the potential for contamination from the Gllaone Gas Station located along the main highway. The site owner indicates that there is a series of underground storage tanks (USTs) located east of the main station building. The tanks are old and there is a risk that they may be leaking petroleum hydrocarbons (gasoline and diesel) into the subsurface and contaminating groundwater. Residents are concerned that gasoline is being transported through the shallow, unconfined sand aquifer toward Lake Navlis. You will focus first on the partitioning of BTEX compounds (benzene, toluene, ethylbenzene, and xylenes) in the shallow subsurface. You will then examine some hypothetical contaminant migration scenarios. Remember you have little data to conduct your investigation. There are no shallow wells near the Gllaone station and the only borehole record nearby (WSW-1) showed no evidence that the unconfined sand aquifer was present near the station. Based on previous experience, you suspect that LNAPL (light non-aqueous phase liquid) is present at the site as either residual or free-phase contamination, but again have no data to confirm your suspicions. Relevant Parameters You can use the parameter values from the previous problem sets in your analysis. They are summarized below. Unconfined Sand Aquifer • Horizontal gradient = 0.012 • Hydraulic conductivity = 1.3 × 10-4 m/s • Effective porosity = 0.25 • Average groundwater velocity = 0.54 m/d or 200 m/yr • Dispersivity values (estimated): ?Longitudinal (?L) = 9 m ?Horizontal Transverse (?TH) = 0.5 m ?Vertical Transverse (?TV) = 0.04 m Questions to Answer 1. Consider a spill of gasoline into the subsurface and the subsequent partitioning of the compounds from the LNAPL into other phases (water and vapour). The mass fraction of various chemical constituents in a Gllaone gasoline sample is provided in Table 1 above. The average molecular weight of the “other hydrocarbons” is estimated at 105 g/mol and the average bulk density of the LNAPL mixture is 0.87 g/cm3. The chemical properties for the BTEX components should be taken from your notes (Lecture 16 – NAPL’s). a) Calculate the mole fraction for each of the BTEX components in the gasoline sample.  b) Assuming equilibrium partitioning between the NAPL-air-water phases in the vadose zone, calculate the aqueous phase concentrations (in mg/L) and vapour phase concentrations (in mg/L) for each BTEX component. c) The values from (b) are theoretical estimates. Comment on whether you think the actual concentrations in the vadose zone would be higher or lower than the predicted values. Briefly explain your answer. d) If benzene was detected in shallow groundwater at a concentration of 5.9 mg/L at this site, is it likely that residual LNAPL is present in the subsurface nearby? Briefly explain. (5 marks) 2. If LNAPL reaches the water table it could potentially contaminate the unconfined aquifer. Because toluene has the largest mass fraction of the BTEX components, you are asked to determine the potential for toluene contamination in groundwater resulting from LNAPL reaching the water table. a) Calculate the distribution coefficient and retardation factor for toluene. You estimate that a sand aquifer would have a porosity of 0.25, the bulk density of 1.85 g/cm3, and an organic carbon fraction of 0.15%. Use the following relationship between log Kow and log Koc. log K_oc = 0.989 ?log K_ow ? 0.346 b) Consider you had exactly one cubic meter of the sand aquifer material. If the LNAPL was present at a residual saturation of 0.1 and the water saturation was 0.9, calculate the mass of toluene in the LNAPL, the dissolved, and the adsorbed phase. Where is the bulk toluene mass stored in the cubic meter of soil? What percentage of the total mass is in each of the NAPL, dissolved, and sorbed phases? c) Given the effective solubility calculated for toluene, what volume of water would have to flow through the same 1 m3 soil block in order to completely dissolve the remaining NAPL, assuming that you can apply the effective solubility to the whole NAPL?  3. Now consider the transport of benzene in the horizontal direction through the unconfined sand aquifer. You will use BIOSCREEN-AT to perform some calculations of transport and biodegradation using a variety of hypothetical scenarios. Assume the size of your contaminant source perpendicular to groundwater flow is 20 m wide and 3 m thick. Use the effective benzene solubility determined in Question 1b for the source concentration. Use a retardation factor of 1.6 for benzene. For questions dealing with instantaneous biodegradation reactions, background concentrations of important chemical redox species are provided in Table 2. Remember to show any output that is relevant to your results. Choose plots that help to explain your answers. a) Start by examining the plume with no biodegradation using BIOSCREEN-AT. How long will the plume be (in meters) after 5 years of transport? Evaluate the length of the plume using a concentration contour limit of 0.005 mg/L, which is the drinking water guideline for benzene. Plot the benzene concentration along the plume centerline. b) With no biodegradation, compare the shape of the benzene plumes using the BIOSCREEN and BIOSCREEN-AT models. How and where are they different? Examine the benzene concentrations measured 500 m downgradient of the source on the plume centerline as well as 25 m away from the centerline. How do they compare? c) Now consider first-order biodegradation. Under aerobic conditions, benzene has a half-life of approximately 3.5 days, whereas under anaerobic conditions its half-life is close to 230 days. Using BIOSCREEN-AT, calculate how long the benzene plume will be after five years for both aerobic and anaerobic conditions? Has either plume reached steady state after five years’ time? Remember to calculate plume size using 0.005 mg/L as your cutoff. d) Now consider an instantaneous biodegradation reaction using BIOSCREEN. How long is the instantaneous reaction plume after five years? Compare the concentrations within the instantaneous reaction plume to the aerobic first-order decay plume (t1/2=3.5 days). Be sure to examine differences in concentration close to the source (set “modeled area length” to 150 m). Which case results in more biodegradation? e) You decide to evaluate the potential for enhancing biodegradation by adding more electron acceptors. You decide to add nitrate fertilizer so that nitrate concentrations in the area increase from 5 mg/L to 40 mg/L. Considering only the instantaneous reaction model, does increasing the background nitrate concentration appreciably change benzene concentrations in the plume? Does the plume reach steady state any faster by adding nitrate? Comment on why you think the nitrate is or is not effective at enhancing biodegradation?

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