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Ground Sampling in an Active Industrial Waste Landfill

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April/May 2011

This article provides a summary of experiences using various sampling methods/equipment to conduct groundwater sampling at an active industrial waste landfill and describes the results of using a Proactive® Environmental Products SS-Monsoon® Pump. For client confidentiality reasons, various site-specific identifiers have been omitted or changed.

Groundwater monitoring is conducted at the subject facility to satisfy regulatory requirements. The facility is located in a region where the depth to groundwater is often over 100 feet below the ground surface. No releases of constituents of concern (COCs) have been reported at the facility. Collection of high quality groundwater samples is a high priority for the facility since any analytical result that did not accurately reflect actual groundwater quality could potentially lead to significant costs incurred for increased monitoring, well installations, etc. RCRA metals are one of the analytical parameters for the facility and it was recognized that the turbidity of the groundwater samples could affect the analytical results. Therefore, the collection of low turbidity samples, obtained by the most cost-efficient means, has been a primary goal in the development of sampling protocols for the facility.

The landfill is located in central South Carolina, in the Carolina Sandhills region of the Upper Coastal Plain Physiographic Province of South Carolina, which is characterized by relatively thin sedimentary sequences of Mesozoic to Quaternary aged clays and sands that unconformably overlie pre-Mesozoic basement rocks. The Sandhills region is located along the Piedmont Physiographic Fall Line and stretches from eastern North Carolina, through South Carolina, into central Georgia. An estimated 200 feet of unconsolidated sediments underlie the site. The subsurface site geology consists primarily of fine to coarse angular sand zones interbedded with kaolinitic clay lenses. The clay lenses vary in thickness from thin laminations to massive lenses several inches to several feet thick. The permeable sand zones typically are poorly sorted and contain various amounts of silts and clays. Depth to groundwater at the site ranges from approximately 100 to 115 feet below ground surface.

Groundwater samples are collected semi-annually from six wells and analyzed for the following parameters:

  • Volatile organics
  • Semi-volatile organics
  • RCRA metals (total and dissolved)
  • PCBs
  • Ethylene Glycol

Because of the presence of significant amounts of clays in the subsurface, the potential affects of sample turbidity on analytical results is a concern at the site. Metals, most notably arsenic, barium, and lead commonly are naturally occurring in the region and are frequently detected in turbid or unfiltered water samples. Therefore, low turbidity samples minimize the potential detections of metals.

Six wells, MW-A – MW-F, comprise the monitoring well network for the landfill. The wells were constructed using 2-inch PVC well casing and screens. Two of the wells were installed approximately 20 years ago, while the other four wells were installed approximately eight years ago. Due to very low recharge rates, two of the wells (MW-B, MW-C) are sampled using bailers. The remaining four wells are sampled using a submersible pump. The pumped wells are sampled once the well is producing water with a turbidity of less than 5 Nephelometric Turbidity Units (NTU), essentially drinking water clear. Metals samples are collected at the bailed wells from the first bail.

For comparison purposes both total metal and field-filtered metals samples are collected. Historical Groundwater Sampling Protocols The two older wells (MW-A, MW-B) had historically been sampled by bailing with little attention to sample turbidity. Once low turbidity samples were desired, the metals samples for these wells were collected the day following the well purging, to allow for the water column in the well to clear somewhat. This procedure produced lower turbidity samples, but NTU values of less than 100 were still rare. Additionally, this procedure required two mobilizations to the site.

The four newer wells were initially developed and sampled using a bladder pump system. Compressed air for the bladder pump was supplied by a portable gasoline engine powered, oil-less compressor. The bladder pump system was capable of pumping water from the approximate 100-foot depth to groundwater but was somewhat cumbersome to install and operate. Occasionally the check valves in the pump would be stuck partially open by sediment. This required removal of the pump from the well and pump disassembly to correct the fouling. This then required the pump and air tubing to be decontaminated each time prior to reinstalling the pump in the well. Flow control of the bladder pump was somewhat difficult to regulate as both the pump fill time and discharge time had to be adjusted. Whenever the pump became fouled or the flow rate needed to be adjusted, this resulted in a disturbance to the well that often produced increased turbidity and lengthened the purging time. Insertion and removal of the bladder pump also appeared to surge the water column in the well due to the pump’s relatively large displacement size, resulting in increased turbidity and longer purging times. Because of similar occurrences using a bladder pump at another site in the region, it was decided to sample using a submersible centrifugal pump.

A Proactive Engineered Plastic Monsoon® pump was selected for use as this pump did not require the use of a gasoline generator that some other systems required. This pump was equipped with a Low Flow Power Booster 2 controller to regulate flow rates and operated from a 12-volt vehicle battery. This system was used for several years and effectively pumped water from the 100-foot depth at the site. The Monsoon pump system was dedicated to only three different sites that had either no COCs detected or only low level detections, in order to minimize the potential for cross-contamination. Flow rate control was adjustable but actual flow rates needed to be determined using simple bucket tests. Onsite and regional experience indicated that a flow rate of approximately 0.5 gpm was the optimum flow rate to achieve low turbidity samples with turbidity values of less than 5 NTU. Site experience indicated that some turbidity was generated each time the Monsoon pump was installed in a well, even with its smaller size relative to a bladder pump.

This system lasted several years. Pump failure occurred during one sampling event. Having no backup pump, the sampling event was discontinued and resumed using a rental Proactive SS-Monsoon® which functioned well, but the required additional mobilization to complete the sampling event results in additional costs that could not be passed along to the client. A second Engineered Plastic Monsoon® pump was subsequently acquired and like the previous system was dedicated to only select sites. This system also lasted several years but also failed during a sampling event and required a second mobilization. After reviewing potential pump replacements a Proactive® Environmental Products SS-Monsoon® Pump, equipped with a Low Flow Power Booster 2 LCD controller to regulate flow rates was acquired to complete the sampling event. The pump and power cord was supplied on a steel reel. This system effectively pumped from the 100-foot depth to water. The LCD readout on the low flow controller allowed for a visual check of the voltage being supplied to the pump. A series of flow rate bucket tests were performed and the voltage noted for different rates. Once the flow rate had been adjusted (and verified by bucket tests) to the approximate 0.5 optimum pumping rate for the site, and the voltage noted, the pump could be installed in other wells onsite with similar depths to water and set to the same voltage. A 0.5 gpm flow rate was verified in additional wells by performing bucket t
tests. Once a few tests are performed, the LCD feature allows the user to develop a flow rate chart for a site based upon the voltage applied to the pump and eliminates or minimizes the need to independently verify pumping rates.

This system also came equipped with a field replaceable pump motor, which, should pump motor failure occur during a sampling event, eliminates second mobilizations (and their costs) to complete the sampling.

Historical Groundwater Sampling Results

Groundwater sampling results for the site have remained relatively consistent through time regardless of sampling techniques and equipment because the standard for sample collection was turbidity values of 5 NTU. No COCs have been detected at the site except infrequent low level detections of common laboratory contaminants and low-level metals detections attributed to naturally occurring COCs. Records for the period when bailing was used to sample the older wells is not currently available. Results for organic COCs have never been presented in tabular form because there have been so few detections. However, metals analytical results for the most recent seven years of sampling are available.

During this time either the Engineered Plastic Monsoon® pump or the SS-Monsoon® pump was used for sampling at the site. A review of the analytical results indicates that low turbidity samples acquired through either pump system provided similar results with only low-level concentrations of metals detected. These detections appear to be naturally occurring. A comparison of the results for filtered versus the unfiltered samples indicated that the acquisition of low turbidity samples using submersible 12-volt centrifugal pumps is an effective sampling method.

Comparison of Sampling Methods

Various methods have been utilized to achieve low turbidity groundwater samples at the subject site. The least effective method is through the use of hand bailing. Because even minor well disturbances increase sample turbidity at the site, even purging wells by bailing and waiting until the next day to collect metals samples rarely produced turbidity less than 100 NTUs. Sampling for metals by collecting the metals sample in the first bail can provide samples with turbidity at less than 5 NTU, but any mistake that causes a disturbance in the water column will raise turbidity levels. Feeling for the top of the water column at a depth of 100 feet requires a delicate touch that not all operators can master. Additionally, some regulators may not approve of this sampling method.

Use of a bladder pump system can produce low turbidity samples. However there appear to be several drawbacks to the bladder pump system, especially for sites similar to the subject site that have significant amounts of clays in the aquifer. From a logistical standpoint the bladder pump system needs an air or inert gas supply. This requires either using compressed gas bottles or using a compressor. Compressors must be oil-less and are usually gasoline powered, which raises the risk of cross contamination of VOCs to sampling equipment. From an operational standpoint, a bladder pump system used in formations with large amounts of silts and clays may experience fouling of the check ball seats requiring removal of the pump from the well with resultant delays for servicing and additional decontamination. The greater displacement size of a bladder pump also creates significant internal well disturbances increasing the likelihood that formational and sand pack fines and clays may become suspended and raise turbidity levels. Bladder pump flow rate adjustment can be tedious, as two separate adjustments are often needed. Additional costs may also be incurred if both air supply tubing and pump discharge tubing need replacement or decontamination between wells. Bladder pumps usually require disassembly for field decontamination. Based upon experience at this and similar sites, the bladder pump system appears to require much more labor and other costs to achieve the desired results.

The Proactive Engineered Plastic Monsoon® pump was effective in producing low turbidity groundwater samples. Compared to a bladder pump, the Monsoon’s smaller size lowers well disturbance effects when installing the pump in the well. Flow rate could be adjusted with the turn of one dial, but flow rates needed to be verified by bucket tests. Only a single polyethylene tubing is needed with the Monsoon pump. For sites with recurring sampling, tubing can be dedicated to each well, minimizing overall costs and decontamination time. The smaller size of the Monsoon pump allows for easier field decontamination.

The pump can be placed in a container of clean water with Liquinox and the solution pumped through the pump, followed by pumping clean water, then deionized water through the pump. Failure of a Monsoon pump without a readily available backup pump may result in sampling delays and additional mobilization costs. The Proactive® Environmental Products SS-Monsoon® Pump, equipped with a Low Flow Power Booster 2 LCD controller, was also effective in producing low turbidity groundwater samples. However, the SS-Monsoon appears to have several advantages as a sampling system relative to the other equipment used at the site. Relative to the bladder
pump system, the SS-Monsoon has the same advantages as the Engineered Plastic Monsoon pump has versus the bladder pump system, but with the SS-Monsoon’s size being smaller than the Monsoon, even less well disturbance effects are likely during installation of the pump in a well. Like the Monsoon, the SS-Monsoon’s flow rate can be adjusted with a single dial. However, the LCD readout showing pump voltage allows the user to reproduce flow rates without bucket tests or other types of flow verification when pumping water from similar depths. A user should be able to construct their own flow rate curves/charts for various water depths based upon pump voltages. The replaceable pump motor in the SS-Monsoon provides a level of confidence that a scheduled sampling event can be completed on time with no additional mobilizations required should a pump motor fail. Decontamination of the SS-Monsoon was quite simple.

Summary

Various sampling equipment and techniques can be used to collect representative groundwater samples with low turbidity. For groundwater sampling in unconsolidated formations, especially those with deep water tables and a high percentage of clays and fines, a submersible centrifugal pump appears to be a highly effective method to collect samples. Based upon the experiences at the subject site, the Proactive® Environmental Products SS-Monsoon® Pump, equipped with a Low Flow Power Booster 2 LCD controller, appears to be the most reliable and cost effective system tested for sampling in similar environments.

About the Author William D. Wood, Jr., P.G. has over 20 years experience in environmental consulting and is a Professional Geologist registered in South Carolina and Georgia, and a Licensed Geologist in North Carolina.

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