THE SKINNY ON GROUNDWATER STATISTICS

Transcription

1 THE SKINNY ON GROUNDWATER STATISTICS PRESENTED BY KIRK CAMERON, PH.D. MACSTAT CONSULTING, LTD 27 years of environmental statistical consulting to EPA, DoE, DoD, AFCEC, EPRI, private & public entities Primary author of Unified Guidance Consultant to Sanitas; developer of GTS for groundwater monitoring optimization Expert statistical witnessing; long history of teaching/ training QUICK PROFESSIONAL CHEATSHEET WHY STATISTICS? WHY HERE? WHY NOW? Statistical monitoring long required under RCRA New CCR Rule adopts older RCRA groundwater monitoring regs almost wholesale Points to EPA s Unified Guidance (2009) to figure out (1) what to do; Strategy (2) how to do it UNIFIED GUIDANCE A MUST READ!! Basics Tests Unified Guidance designed as comprehensive statistical resource Conceptual framework; statistical basics Regulatory review Testing strategies tailored to RCRA (and CCR) Worked out examples of each technique Guidance on how to build a proper monitoring program

2 THREE-PHASE STATISTICAL PROGRAM Detection: comparing data to BG Assessment: comparing data to GWPS RCRA Subtitle C & D Programs; CCR Corrective Action: comparing data to clean-up standard Appropriate statistical strategy changes depending on phase Big Question: Are chemical concentrations statistically larger than background? GOAL OF DETECTION Test each constituent on list Wells located around/near compliance boundary Statistically significant increase (SSI) over background? Interwell or intrawell? Cum sociis natoque penatibus magnis parturient montes. COMPARE BG VS. COMPLIANCE WELL DATA Unified Guidance lays out three recommended primary techniques: Prediction Limits Control Charts Trend Tests NOTE ON INTRAWELL TESTING Intrawell tests often needed due to significant spatial variation Naturally occurring analytes often vary by location BUT, be careful of synthetic spatial differences! Intrawell BG built from early events at each compliance well Each well supplies its own BG Intrawell BG tested against more recent data from same location Guarantees apples-to-apples comparison! Concentration (ppm) Boxplots of BG Data Well A Well B Well C Well X Well Y

3 DETECTION STRATEGIES DETECTION STRATEGIES (CONT.) Prediction Limits Limit computed from BG BG can be upgradient (interwell) or intrawell SSI only when compliance data exceed prediction limit (and confirmed by retesting) Recommend 8-10 BG observations 09:30 05:20 AM 09:45 PM Nickel Concentration (ppb) BG Compliance CUSUM h-limit h = Quarter Control Charts Control chart limit computed from BG Similar to prediction limits (but more visual) SSI when compliance data or cumulative sum (CUSUM) exceed control limit Same basic data requirements as prediction limits DETECTION STRATEGIES (CONT.) Trend Tests Test for statistically significant positive slope Increasing trend = SSI (indicates GW deterioration) No explicit retesting, but can be added Prefer at least 6-8 observations Trend tests are inherently intrawell; checks for changes over time at a fixed sampling point BUT WHAT ABOUT ANOVA?? Deprecated in EPA s Unified Guidance No longer recommended for detection monitoring Still useful for diagnostic tasks Identifying spatial variation! AVOID ANOVA AT ALL COSTS

4 ANOVA mentioned in RCRA & CCR Rule, but discouraged in EPA s UG! Why? ANOVA DRAWBACKS Unacceptable per constituent false positive rate RCRA-mandated minimum of 5% Many COCs at α=0.05 leads to very high SWFPR CCR application: 7 COCs, SWFPR 50% per year (semiannual evaluations) NAIL IN THE COFFIN!! Cum sociis natoque penatibus et magnis parturient montes. ANOVA inherently interwell Interpretation often meaningless if natural spatial variation exists What do well-to-well differences represent? GW Flow Coastal River Road Landfill ,000 10,000 SPECIFIC CONDUCTANCE (umhos/cm3) 900 Sea Water Long-term Data (10 yrs) Max = 30,000 Ave = 22,000 STAGE 2: ASSESSMENT WHAT S THE GOAL? ASSESSMENT STRATEGIES Compare compliance data to GWPS GWPS = MCL, ACL, other regulatory standard No direct** comparison to BG If SSI over GWPS? Statistical trigger for corrective action Big Question: are concentrations statistically larger than groundwater protection standard (GWPS)? Confidence intervals Stable compliance data (no trend) SSI only if lower limit exceeds GWPS Implies population mean > GWPS Minimum of 4 values; 8-10 recommended

5 date ASSESSMENT (CONT.) chloride Chloride Confidence Band No Adjustment Confidence bands Trending data Combines sliding confidence interval around trend line Point-in-time comparison; SSI when lower band first exceeds GWPS 8-10 values strongly recommended GOAL OF CORRECTIVE ACTION Compare remedial data to clean-up goal or GWPS Must show statistically significant decrease (SSD) below clean-up goal May also need to show longer-term attainment (i.e., projected trend below clean-up goal) CCR Escape Clause if three consecutive years below GWPS Detection Big Question: Are compliance concentrations below clean-up standard? CORRECTIVE STRATEGIES Confidence intervals (again!) Stable compliance data (no trend) SSD only if upper limit below clean-up goal Implies population mean < goal Minimum n = 4; 8-10 recommended WHAT IF TRENDING DATA? Cum sociis natoque penatibus et magnis parturient montes. Confidence bands (in reverse) Point-in-time comparison SSD when upper band first drops below clean-up goal At least 8-10 observations strongly recommended TCE Concentration (ppb) Clean-Up Goal 90% Conf. Band Theil-Sen Line TCE Month

6 SELECTING THE RIGHT STUFF WHICH TEST/STRATEGY TO USE? A No single test always appropriate for every COC D Need to meet test assumptions for defensible results B May need mix of interwell/ intrawell E Re-examine statistical program every 3-5 years Stage of Monitoring Compare against BG or GWPS? Need to account for trends? Already contaminated? Hydrogeologic Conditions Natural Spatial Variation? Potential for migration from offsite? Interwell or Intrawell? Data Characteristics Quantity; sampling frequency Statistical distribution Lots of NDs? Seasonality? C May need mix of parametric/non-parametric F Accumulated data may allow for better tests/approach Appropriate Tests & Proper Interpretation Lead to Good Decisions & Better Science THANK YOU. For more information: kcmacstat@gmail.com