Low-Flow Ground-Water Sampling Latest Research and Equipment Options

Transcription

1 Low-Flow Ground-Water Sampling Latest Research and Equipment Options David Kaminski QED Environmental Systems Inc. Ann Arbor, MI - San Leandro, CA Copyright QED Environmental Systems, Inc. 2017; all rights reserved. The information contained within this document may not be published, broadcast, rewritten or otherwise distributed without prior written authorization from QED. Today s Webinar Topics Early well purging research and guidelines Limitations of traditional purging/sampling methods Purging evolution: Low-flow purging and sampling Advantages of low-flow purging and sampling Low-flow purging and sampling application guidelines Other low-flow application issues What do low-flow samples represent? Where should the pump intake be placed? Is there a minimum purge volume? Is there a screen length limit for low-flow sampling? Sampling pump options for low-flow purging/sampling Summary and Q&A Session 1

2 2 Early purging research resulted in guidelines to remove stagnant water from the well The rule of thumb was 3 to 5 well volumes prior to sampling to get formation water Low-yield wells were evacuated and sampled upon recovery, typically within 24 hours Little concern was given to how purging protocols and devices used (e.g., bailers) affected the chemistry of ground water samples What does the sample represent with traditional purging methods? Water Chemically Altered by Gas Exchange (O 2, CO 2 ) Water from other vertical zones enters screen during pumping Normally Immobile Colloids and Sediment Elevate Turbidity NAPL micro-globules are mobilized

3 3 Traditional Well Purging Effects on Sample Chemistry and Quality High purge volume can cause underestimation of maximum contaminant concentrations due to dilution High purging rates can cause overestimation due to contaminant mobilization and increased sample turbidity Dewatering lower-yield wells affects DO and CO2 levels and can increase sample turbidity, both which can affect concentration of metals in a sample (low or high bias) Excessive drawdown can cause overestimation or false positives from soil gas or from mobilization of soil-bound contaminants in the overlying formation or smear zone Increasing turbidity Initial water clear Time 14:01 Time 14:04 Turbidity > 75 NTU; 50 liters total purge volume Time 14:06 Time 14:21

4 Hand bailing and high-rate pumping can elevate sample turbidity Turbidity can elevate metals and some organics bound to solids (e.g., PAHs) Sample filtration adds cost & time in field or lab; some regulatory programs (e.g., CCR rules) require an unfiltered (total) sample for metals Filtration affects sample chemistry Turbid samples that are filtered to remove solids are not the same as low turbidity samples Gibbons & Sara, 1994 found no statistical difference between filtered and unfiltered samples for metal when turbidity is <10 NTU. Various guidance documents suggest 5-20 NTU is acceptable for sampling (e.g., Florida DEP FS2200, 2006; US EPA Region 1 SOP, 2010) Low-flow purging and sampling can solve problems seen with traditional well purging methods Low pumping rate minimizes drawdown, in-well mixing and formation stress, isolates stagnant water above screen Low stress = low turbidity, improved sample accuracy, reduced purge volume Samples represent naturally mobile contaminants, not stagnant water in the well or mobilized contaminants Purge volume is based on stabilization of water quality indicator parameters, NOT a minimum purge volume or purge time 4

5 Concentration in mg/l Lower flow improves sample quality Low-flow purging and sampling controls turbidity and delivers higher quality samples - a clear advantage. Effect of low-flow sampling on data accuracy and precision Island County Landfill - Unfiltered Metals Concentrations - Well E2S Wells purged and sampled with bailers; high turbidity (>100 NTU) Purged with pump and sampled with bailers; varying turbidity (30-50 NTU) Chromium Nickel Changed to low-flow sampling with dedicated bladder pumps 0 Dec-88 May-90 Sep-91 Jan-93 Jun-94 Oct-95 Mar-97 Jul-98 Dec-99 Apr-01 Sep-02 Jan-04 Date 5

6 6 Reduced Purge Water Handling/Disposal Traditional Well Volume Purging Low-Flow Purging Cost Savings with Low-Flow Sampling (From Schilling, 1995) Low-flow Purging Three Well Volumes Purging Analysis: Total Purge Volume (15 wells) 61 gallons 743 gallons Average Volume Purged 3.3 gallons 50 gallons Average Pumping Rate 0.3 GPM 2-5 GPM Average Purging Time per Well 13 minutes 50 minutes Total Purging Time (15 wells) 3.25 hours 12.5 hours Economic Analysis (in US Dollars): Time for Purging Wells (a) $500 $1,875 Disposal costs (b) $1,300 $3,750 Cost per Sampling Event $1,800 $5,625 Annual Sampling Costs (quarterly $7,200 $22,500 sampling) Sampling costs for 30 years $216,000 $675,000 (a) Two-person crew at $150/hr.USD (b) First drum = $1,000; additional drums = $300 (drum = 55 US gallons/208 liters).

7 7 Advantages of Low-Flow Sampling Low-flow is a consistent, performance based standard for purging, rather than an arbitrary rule of thumb It documents purging process for every sample, overcoming factors that can affect required purge volume Low-flow sampling can reduce sampling costs: Direct cost savings - reduced purge water handling & disposal, reduced purging time (in some wells). Sample Quality - reduced turbidity, more accurate dissolved concentrations, and a better estimate of the true mobile contaminant load Indirect cost savings - improved data accuracy and precision (fewer false statistical hits ); better data = better decisions Low-Flow Sampling Application Guidelines The Basics Flow rates must be controlled to pump without continuous drawdown (water level must stabilize) and not increase turbidity. Rates of 200 to 1,000 ml/minute are typical Drawdown is based on well performance during purging, not an arbitrary drawdown limit Indicator parameters are monitored for stabilization to indicate formation water and purging completeness Dedicated sampling equipment is preferred. Portable pumps require larger purge volumes, can increase turbidity and require decontamination between wells, but are still better than bailing or high-rate pumping

8 8 Purging Flow Rates From US EPA, 1996: Typically, flow rates on the order of L/min are used, however this is dependent on site-specific hydrogeology. Some extremely coarsetextured formations have been successfully sampled in this manner at flow rates to 1 L/min. The goal is to achieve a stabilized pumping water level as quickly as possible. This reduces mixing within the borehole, drawing water from the sampling zone Flow rates are established for each well based on drawdown values measured during purging, not an arbitrary value or upper limit Sampling Flow Rates Sampling flow rates less than 0.5 L/min are appropriate. (US EPA 1996) Sample at or below the purging flow rate - the rate can be reduced for sampling, but increasing rate after stabilization could cause mixing of well water and formation water Fill larger sample bottles first, then reduce the flow rate (if needed) for VOCs and any filtered samples (typically ml/min or less) Sampling at very low flow rates for all parameters can extend sampling times unnecessarily don t set purging rate too low!

9 9 Water Level Drawdown Limits From USEPA 1996, Puls and Barcelona: The goal is minimal drawdown (0.1m) during purging. This goal may be difficult to achieve under some circumstances and may require adjustment based on sitespecific conditions and personal experience. Water Level Drawdown Recommendations The recommendation from Puls and Barcelona (1996) has been interpreted as a maximum drawdown limit in some regulatory guidance documents. There is no data to support this or any other arbitrary drawdown limit. A study by Vandenberg and Varljen (2000) shows that the goal is to establish a stable pumping water level during purging, with indicator parameter stabilization following water level stabilization.

10 Drawdown (ft) Sp. Cond. (us/cm) Correlation of Drawdown and Indicator Parameter Stabilization 16 Drawdown and Specific Conductance During Purging St. John's Landfill Well D-2A (Vandenberg and Varljen, 2000) Drawdown Pump Off Specific Conductance (10-minute moving average) Casiing :00 14:30 15:00 15:30 16:00 Time When water level stabilized, indicator parameters (conductivity shown above) and target analytes were also stabilized. 475 Indicator Parameters for Purging Indicator parameters often include ph, temperature, electrical conductivity, DO, ORP (redox) and turbidity. DO and EC are the most reliable indicators, based on published research and field experience. ph stabilizes readily, often shows little change Temperature measured at the well head is affected by sunlight, ambient temperature, and some electric pumps Turbidity doesn t indicate when purging is completed. It should be measured primarily to support sample data and identify excessive pumping/formation stress Stabilization criteria are typically % of readings or a range of units (e.g., mg/l DO, ph units) where percentages are not appropriate. Stabilization occurs when three consecutive readings fall within the criteria. 10

11 Temp, ph, DO, Turb EC, ISE 11 A flow cell isolates water from air, maintaining water chemistry and allowing automated measurement. Open-top flow containers can t achieve accurate values for dissolved oxygen or redox due to rapid gas exchange. Typical Indicator Parameter Stabilization Curves Using Low-flow Purging Time, minutes Temp oc ph DO mg/l Turb ntu EC us ISE mv

12 Other questions and regulatory issues with low-flow purging and sampling Do low-flow samples represent the entire well screen zone, or just a discrete interval? Does the pump inlet location affect sample results? Is purging the well screen zone sufficient to provide a representative sample? Does low-flow sampling work in longer well screens, or is there a screen length limit? What Does a Low-Flow Sample Represent? Empirical studies and modeling simulations show that the entire well screen contributes to the sample Flow into screen is controlled by the geology near the well, regardless of pump position; high K zones contribute more water The actual zone monitored is longer than the length of the screen Same for 5, 10, and 20 foot screens (1.5, 3 and 6 meter screens) Applies to both fully submerged screens and screens intersecting the water table Varljen, et al

13 13 Vertical Distribution of Flux into a 10-foot Well Screen and Effect of Changes in Pumping Rate Varljen, et al Effect of Pump Placement on Vertical Flux Distribution Varljen, et al. 2006

14 14 Effect of Pump Placement on Vertical Flux Distribution Varljen, et al Effect of Heterogeneities on Flux Distribution Pattern Varljen, et al. 2006

15 15 Effect of Heterogeneities on Flux Distribution Pattern Varljen, et al Proportion of formation water in the pumped water, as a function of dimensionless time, based on five analytical and numerical models of wellbore flow Model A = 90% formation water in the pumped water at 2.3 well screen volumes Model B = 90% formation water in the pumped water at 1.8 well screen volumes From: J.M. Martin-Hayden et al., Groundwater, Vol. 52, no. 1, pages (Jan/Feb 2014)

16 16 Content of inflowing formation water and flow lines calculated using finite element modeling Blue indicates inflowing formation water and red indicates pre-pumping well water 90% formation water in pumped water after purging 1.8 to 2.3 well screen volumes From: J.M. Martin-Hayden et al., Groundwater, Vol. 52, no. 1, pages (Jan/Feb 2014) Wellbore Flow Modeling Low-Flow Sampling Implications 90% formation water could be achieved after purging between 1.8 and 2.3 well screen volumes The model assumes that the well bore water DOES NOT represent formation water before purging (worst case scenario) 2 (50mm) x 10 (3m) well screen = liters purge volume Where water within the well screen is some mixture of formation water and well bore water, less purging would be needed Empirical data from published studies is consistent with the model, ranging from 8 20 liters in many wells

17 17 Screen Length Limits Using Low-Flow USEPA s 2002 guidelines limit low-flow purging to wells with screens 10 or less Their reference for this (USEPA, 1996, Puls and Barcelona) DOES NOT SUPPORT THIS LIMIT No other independent data or any other published study is cited to support the limit Some state regulatory agencies have used the USEPA 2002 guidelines to limit use of low-flow purging to well screens no longer than 5-10 feet. Well screen length controversy (Based on USEPA, 2002 RCRA-CERCLA Program Guidelines) Screen = 50% saturated thickness Screen = 4% saturated thickness 30 1, LOW FLOW? YES! LOW FLOW? NO.

18 18 Screen Length Issues and Objectives The issue of well screen length is one of monitoring program objectives and not a sampling method issue Screen length should relate to the saturated thickness and identifiable flow paths and should not be based on an arbitrary design or guideline Studies support using low-flow purging and sampling in well screens to 20 feet (6 meters) or longer. In longer well screens, a multi-port pump inlet design can be used to sample along the entire well screen. ASTM Standard D6771 on Low-Flow Purging and Sampling Originally published in 2002, it was removed from publication in 2010 after the committee members failed to reach a consensus on much needed revisions to the document Currently being revised in Subcommittee D18.21, with a June 2017 publication target Many aspects of the standard follow industry practice and generally accepted procedures, but some proposed guidelines may be onerous and unnecessary: Requirements for purge parameter stabilization could extend purge time If sampling flow rate is reduced, it may require additional purging and re-stabilization of the indicator parameters, adding time to sampling If you are a practitioner or regulator, consider joining ASTM to vote and comment on the standard

19 19 Low-Flow Sampling Pump Options Pumps for low-flow purging and sampling should have: Adjustable flow to match the purge rate to the well yield Ability to lift water from the required depth, including drawdown Minimal sample alteration due to pressure change, heat, etc. Inertial lift pumps won t work - they mix and agitate water column and increase sample turbidity Common purge & sample pumps for low-flow include: Peristaltic pumps also known as tubing pumps Electric submersible pumps most are centrifugal designs Bladder pumps air pressure squeezes bladder to lift water Low-Flow Peristaltic Pumps Can be DC-voltage (battery powered) or AC-voltage (require generator or DC/AC inverter power) Fits any well diameter, including small direct-push wells and multi-level systems Tubing can be disposable or dedicated Suction lift limited to feet (6-8 m) Flexible elastomeric tubing required at pump head can affect sample chemistry through sorption Suction applied to sample can affect VOCs and metals (Devlin, 1983; Barcelona, et al., 1984) some regulators don t allow sampling through the pump for these parameters Battery-powered peristaltic pump AC-powered peristaltic pump

20 20 Electric Submersible Sampling Pumps AC voltage designs require portable generators from 1,000 2,000 watts DC voltage designs use portable battery power or vehicle battery Stand-alone control box for flow rate control and programmable functions Sampling depths range feet (15-60 m) for DC-voltage pumps and 275 feet (84 m) for AC-voltage pumps Flow rate should be controlled by motor speed, NOT with a valve pressure drop across valve could affect VOCs, metals in samples Some designs require cooling shroud over motor to prevent motor damage AC-voltage pump, control box and generator DC-voltage pump and control box Low-Flow Bladder Pumps Bladder pumps offer the best combination of sample accuracy, reliability and depth range for low-flow sampling Accuracy shown to be 99% - 103% of control samples (Waterloo, 1992) Sampling depths to 1,000 (300 m) lift, greater depths with drop tube inlet Wide range of material choices (PVC, stainless steel, PTFE) to match both contaminant chemistry and background water quality QED Well Wizard dedicated models carry a 10-year warranty Sample Pro portable models use disposable bladders & tubing Well Wizard Dedicated Pumps Sample Pro Portable Pumps

21 21 The QED Power Pro ESP Designed for low-flow sampling All-in-one system has controls built into the reel with pump holster and cable roller Brushless DC motor and revolutionary impeller design consumes 50 75% less power for longer battery life and less heat from the motor Simple Flow Control knob to regulate flow from over 1 gpm (4 lpm) to > 100 ml/min Unique ESP Communications Protocol module has integrated In-water Sensor that lights up when pump is submerged Run-Dry Protection to prevent pump damage and limit water level drawdown Sample depths to 150 feet (45m), almost unlimited depth with Drop Tube Inlet kit Power Pro ESP - Cutaway View Pump Intake Pump intake at the bottom, with flow path through the impeller and up over the DC motor no need for external cooling shroud Low-cost replaceable impeller can be serviced by simply removing housing at bottom of the pump no risk of damaging lead wire or connectors Motor module is warranted for 400 hours/1 year of operation guaranteed performance and high reliability. Others are as little as 60 days

22 22 Complete Power Pro ESP System All-in-one system weighs just 32 pounds (14 kg) for easy transport to remote wells Powered by lightweight portable battery or vehicle battery no heavy generators Long-life brushless DC motor, rugged impeller and motor seal for reliability and low maintenance Works with QED s MP20 Flow Cell Low-Flow Sampling Pump Selection Criteria Matrix Sample Accuracy Bladder Pump Electric Submersible (Centrifugal) Power Pro ESP Peristaltic Pump Maximum Depth Ease of Operation Reliability/Maintenance Ease of Field Repair Cost Per Sample Selection Key Excellent Good Fair/Poor

23 Summary Traditional well purging methods can cause significant bias and error in groundwater sample data Low-flow purging and sampling can overcome many of the problems associated with traditional well-volume purging, hand bailing and high-rate pumping, reducing turbidity and improving data precision over time Low-flow purging and sampling will provide a flowweighted average sample from most monitoring wells when used correctly Pumping rate, drawdown level and screen length should be based on hydrogeologic setting and well performance, not arbitrary limits Well screens to 20 feet/6 meters or longer can be sampled with the low-flow method Questions? QED Environmental Systems, Inc. info@qedenv.com Phone: Website: 23