Sampling Theory Basic Considerations of a Sampling Plan

Sampling Theory Basic Considerations of a Sampling Plan • A goal of a sampling plan is to address representativeness • The key questions to a sampling plan: – Where? – When? – How many? 1. Sampling Objectives • • • • Baseline monitoring Trend detection Search for hotspot Margin of error allowable Where to sample if the objective is to determine whether the pipe released organic chlorine compounds? Where to sample if the objective is to determine whether the pipe released organic chlorine compounds? What if the objective is to measure the average organic chlorine concentration in the entire old lagoon? What if the objective is to measure the average organic chlorine concentration in the entire old lagoon? 2. Variability • • • Temporal variation – Representativeness in certain days, weeks, months, years – Only one dimension (1-D) Spatial variation – Representativeness in certain locations – 1-D: e.g., outfall of an industrial wastewater discharge – 2-D: e.g., deposition of lead along a road surface – 3-D: e.g., a large body of water, or a landfill site Usually both need to be considered 3. Cost Factors • Sampling cost • Analytical cost • Fixed vs. minimum cost 4. Nontechnical Factors • • • • Sampling convenience Accessibility Availability of resource Regulations (e.g., EPA has specific sampling guidelines for Superfund investigations) Sampling Approaches • • Judgmental Sampling Designs – Based upon expert knowledge or professional judgment Probability-based Sampling Designs – Apply sampling theory and involve random selection of sampling units – Each member of the population from which the sample was selected has a known probability of selection Sampling Approaches Judgmental Sampling Conditions where judgmental sampling could be suitable: • Relatively small-scale features or conditions are under investigation. • An extremely small number of samples will be selected for analysis/characterization. • There is reliable historical and physical knowledge about the feature or condition under investigation. • The objective is to screen an area(s) for the presence or absence of contamination at levels of concern, such as risk-based screening levels (note that if such contamination is found, follow-up sampling is likely to involve one or more statistical designs). • Schedule or emergency considerations preclude the possibility of implementing a statistical design. Case 1: Area Impacted by Contamination Can Be Visually Discerned An active manufacturing facility is being sold, and the prospective purchaser is conducting an investigation to characterize existing environmental conditions and potential associated liability. One feature being assessed is an approximately 500 m2 fenced area where drums of an aqueous cupricchloride waste are stored. When released, the waste stains the soil blue-green. Eight irregularly shaped blue-green stains are identified ranging in size from about 10 square centimeters to a square meter. The stains are thought to be a result of relatively small releases that occurred as waste was poured into drums at the storage area from smaller containers filled at the facility’s Satellite Accumulation Areas. Case 2: Potential Location of the Contaminant Release Is Known An abandoned textile mill is being investigated as a Brownfields site, and one previous employee was located who gave a reliable account of site features and activities. Through the interview, it was found that there is a 30 meter long drain pipe that carried a variety of wastes from one of the site factories to a leach field adjacent to the building. The drain pipe is accessible under a grating installed on the basement floor of the factory, and several of the joints between the 3 meter length pipe segments appeared either loose or slightly separated. Case 3: Visual Judgmental Sampling A rural county enforcement officer has just started his job a few months ago in an area that he has never lived before. One day when he was tramping along a creek, he found some dark spots in the stream sediment. He believes that there are contaminations in those spots and wanted to find out what that is. Simple Random Sampling • Definition • Appropriate when the population being sampled is relatively uniform or homogeneous • Benefits and limitations: – Avoid selection bias to a degree, provided that the sample size is not extremely small (for example, 20 observations or more) – However, the sample points could still be unevenly dispersed in space and/or time – Ignore all prior information, or professional knowledge, regarding the site or process being sampled – Less efficient • Not commonly used in practice Stratified Sampling • Definition – Divide the population into nonoverlapping strata – Within each stratum, a simple random sampling is employed – The area within each stratum is as homogeneous as possible • Can be more precise and more cost effective than simple random sampling Stratified Sampling • Strata: groups of the population with certain characteristics – Temporal: days vs evenings, weekdays vs weekends, four seasons, etc. – Spatial: rainfall distribution, soil characteristics, age of population, etc. • Need reliable prior knowledge of the population in order to effectively define the strata and allocate the sample sizes • Improper information used to set up the design can reduce precision/increase cost. Stratified Sampling An investigator wants to estimate the average concentration of arsenic in the surface soil around the smoke stack at a hazardous waste incinerator facility to determine if the soil has been contaminated above the naturally occurring concentrations of arsenic for the region. Samples are to be taken within 500 meters from the smoke stack. Information gathered from prior studies indicates that the concentration of arsenic will be higher in the area along the prevailing wind direction and that the variability of the concentration of arsenic in the soil will be higher for clayey soils compared to sandy soils. Systematic/Grid Sampling • Definition – Samples collected at equal intervals over time or space • Good for uniform coverage, ease of use, better precision, and important features of the population being sampled will not be missed • Useful when the goal is to estimate spatial or temporal correlations, to identify a pattern, or to locate a “hot spot” • Limitations – Less efficient – Improper design of grid size Adaptive Cluster Sampling • Definition – Samples are first taken using simple random sampling – Additional samples are taken at locations where measurements exceed some threshold value • Relatively inexpensive, rapid measurements compared to systematic sampling • Useful for estimating or searching for rare characteristics in a population • Enable delineating the boundaries of hot spots Adaptive Cluster Sampling Example • • • A historical nuclear liquid waste impoundment overflows into an adjacent field, where the flow separated into multiple distinct channels before discharging into a river. The outflow has been shut off for 10 years, the field has been paved into a parking lot, and a new building has been proposed for that parking lot. There is no available information indicating the former locations of the flow channels (for example, no aerial photographs or surveys). Construction of a new building could expose workers and future building inhabitants to contamination. Considering that the contamination is likely to be clustered within the former flow channels and that little prior information is known about the specific locations of the channels (the area cannot easily be stratified), adaptive cluster sampling is an ideal sampling design for this situation. A grid would be established across the parking lot. An initial random sample would be collected, and wherever the concentration or radioactivity exceeds threshold values, neighboring locations would be sampled until the entire distribution of contamination in the field is characterized. • • Ranked Set Sampling • • Combines simple random sampling with professional judgment or surrogate measurements Sampling design – Identifies sets of field locations – Utilizes inexpensive measurements to rank locations within each set – Selects one location from each set for sampling Highly useful and cost efficient in obtaining better estimates of mean concentration levels Only applicable when the cost of locating and ranking locations in the field is low compared to laboratory measurements Example Suppose a future residential area is suspected of having lead concentrations in surface soil that exceed background concentrations. Prior studies have shown that x-ray fluorescence (XRF) measurements of lead in soil obtained using a hand-held in-situ detector closely correlate with laboratory measurements of lead in soil at the same locations. Furthermore, it was determined that the cost of taking the XRF measurements in the field was very low compared to the cost of laboratory measurements for lead. Composite Sampling • Definition – Volumes of material from several of the selected sampling units are physically combined and mixed in an effort to form a single homogeneous sample, which is then analyzed • A primary goal is to reduce cost by having fewer analyses • Composite sampling has historically been used mainly for estimating a mean • It can also be used to estimate the proportion of a population that has a particular trait Choosing the Appropriate Sampling Design for Your Problem Choosing the Appropriate Sampling Design for Your Problem Choosing the Appropriate Sampling Design for Your Problem Choosing the Appropriate Sampling Design for Your Problem Choosing the Appropriate Sampling Design for Your Problem Choosing the Appropriate Sampling Design for Your Problem Choosing the Appropriate Sampling Design for Your Problem Choosing the Appropriate Sampling Design for Your Problem Choosing the Appropriate Sampling Design for Your Problem Choosing the Appropriate Sampling Design for Your Problem Choosing the Appropriate Sampling Design for Your Problem Choosing the Appropriate Sampling Design for Your Problem You are asked to estimate the weekly average concentration of SO2 emitted from a stack in a coal power plant. Four alternative sampling plans were proposed to take exhaust gas SO2 samples at the outlet of the stack. a) Randomly collect two samples everyday for seven days b) Collect two samples per day for seven days, sampling interval is 12h (9am and 9pm) c) Collect two samples everyday for seven days. One sample is randomly collected during the day time, and the second sample is randomly collected during the night shift d) Same as c), but four random samples are collected during the day shift and three random samples are collected during the night shift Sampling Techniques Estimate Sample Numbers • Scientific reliability vs. resource availability • Most conservative: simple random sampling • EPA Equation (typically used for solid samples) • Examples An abandoned waste pile needs to be sampled and analyzed due to recent complaints from local residents. Historical data show that the wastes were mainly from a solvent recovery facility in 1970s with a PCB concentration of ~0.70 ppm (average of 5 samples) and standard deviation of 0.12 ppm. The regulatory soil screening level for PCB is 0.74 ppm. Use a 80% confidence level. Estimate the number of samples required. Trial-and-error procedure Historical data from a contaminated site suggested Hg concentrations in the range of 2-20 ppb, and a standard deviation of 3.25 ppb. A thorough survey is needed for a planned remediation of this site. Estimate the number of samples required so that the sample mean would be within 1.5 ppb of the population mean at a 95% confidence level. General Rules for Sampling Sequence • From the least to the most contaminated sampling locations • Water first and then sediment • From downstream to upstream • From shallow water to deep water Sample Amount • Should be sufficient for lab analyses and quality control (duplicates and spikes) • Higher amount leads to higher storage, transportation and disposal costs • Water: Varies from 5mL for TPHs, 100mL for metals, and 1L for pesticides – Generally collect 3-4 times the volume required by standard methods • Soil: Usually 5-100g for contaminant analysis; 200g for full characterization • Air: Usually >10 m3 for ambient air concentrations; lower for smoke stack emission samples Sample Preservation • Purpose: Minimize contaminant changes between collection and analysis • Three approaches – Refrigeration: slow down all loss processes – Use of proper sample container (material type, headspace): preserve photosensitive chemicals, prevent volatilization and sorption – Addition of preserving chemicals: essential for reducing chemical reactions and bacterial degradation Maximum Holding Time (MHT) • The length of time a sample can be stored after collection and prior to analysis • Vary by analyte, sample matrix, and analytical method Selection of Sample Containers Water Samples • Glass vs. Plastics: – Glass may leach boron and silica, metals may stick to walls – Glass is generally used for organics and plastic for metals, inorganics and physical properties – For trace organics cap and liner should be made of inert materials (Teflon) • Headspace vs. no Headspace: – No headspace is allowed for VOC samples – Oil and grease should only be half-filled in wide mouthed glass bottles • Special containers: – e.g. BOD/DO bottles and VOC vials 1-33 Standard Methods (1998) • Soil Samples – Low temperature storage is mostly sufficient – No preservatives except ethanol or sodium bisulfite for VOC analysis (Popek, 2003) Sampling Equipment Soil and Sediment Sampling • Both sediment and soil samples need to be treated with homogenizing, splitting, drying and sieving • Horizontal (grab) or vertical (core) sampling • Composite sampling is common (except for VOCs) • Nonsoil/sediment or nonsieved materials should be noted, not discarded • Sediments from lakes, ponds and reservoirs should be collected at the deepest point – contaminants tend to concentrate in fine grained material in depositional zones • Scoop or trowel for surface soil samples Soil sampling Hand Corer • Depth: 1-10 ft • Preserves vertical integrity Auger • Depth: 3 inch – 10 ft soft soil samples • Hand auger • Machine driven auger • Soil disturbed Split spoon sampler • Excellent depth range • Good for hard soils Sediment sampling Grab sampler (Ekman, Ponar, Van Veen etc.) Sediment core sampler Hazardous Waste Sampling • • • • Sources: drums, storage tanks, lab packs, waste impoundments, waste piles, debris Chemical Drums – Research documentation (labels etc.) for health and safety precautions – Use proper protective equipment – Unknown wastes should be opened remotely – Should not be moved since some chemicals are shock-sensitive, explosive or reactive – Sample each phase separately Waste Impoundments – Contaminants will be stratified by depth, also horizontally from point of entry Surface Sampling – Wipe, chip and dust sampling Grain Thief Coliwasa Surface Water and Wastewater Sampling • Small streams ( 10 NTUs) – Specific conductivity ± 3% – ORP ± 10 mV – pH ± 0.1 unit – Temp. ± 0.1 oC Cross contamination Bailer • Generate turbulence • Exposed to atmospheric O2 • Not suitable for VOC tests Peristaltic Pump • Dedicated tubing • Suitable for wells with small diameters • Depth limitation of 25 ft • Potential loss of VOCs Bladder pump • Suitable for VOC sampling • Sample deep water of ~100 ft Air and Stack Emission Sampling • Concentrations for most atmospheric pollutants are very low • Analysis of organic compounds requires huge volumes • Large variation in analyte concentration due to changes in meteorology • Meteorological parameters must be noted Indoor Air Sampling • Ventilation systems can alter air flow and add pollutants • Sampler location will influence the results obtained • Household chemicals can add compounds to the air Collection of air in a container, e.g., canister or Tedlar bag Filtration of air using filter cassette Chemical absorption through the bubbling of air in a liquid medium Chemical adsorption Biological Sampling • Species availability – Insufficient sample size may result in invalid statistical inference • Sampling protocol needs to account for size differences between species, tissue differentiations, growth stage, and habitat • Susceptible to decomposition of organic analytes Passive Sampling • • • • • • Based on free flow of analyte molecules from the sampled medium to a collecting medium Compact, portable, unobtrusive, and lowcost Time-averaged concentration Requires no supervision, is noiseless and can be used in hazardous environments Amenable to personal monitoring (breathing zone), indoor air analysis, and outdoor ambient air analysis Cannot be used for acute analysis Quality Assurance and Quality Control What is Environmental Sampling and Analysis? – Planning (setting objectives) – Collecting samples (sampling) – Analyzing the samples – Determining if the data/information meets objectives Information must be precise and accurate enough to meet project objectives. The Importance of Data Quality • Scientific Reliability • Your measurements can accurately reflect the content of sample • Legal Defensibility • Withstand any reasonable challenges related to the veracity, integrity, or quality of your measurements • Misconducts = criminal acts! Chain-of-custody form Good documentation • Photos • Lab notebook • QA/QC reports • Analyst records Lab notebook Traceability throughout the entire project QA/QC reports QA/QC System • Developed in 1920s by Walter Shewhart for manufacturing products • Widely app…
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