Unit 12 : Advanced Hydrogeology

1. Unit 12 : Advanced Hydrogeology Contaminant Hydrogeology

2. Contamination Problems • So far we have concentrated on physical and chemical processes of mixing dissolved species in groundwater systems. • Most of the discussion has largely avoid issues of organic non-aqueous phase liquids (NAPLs) and multiphase flow. • In this section we will discuss practical problems of groundwater systems where a vapour phase and more than one liquid phase may be present.

3. NAPLs • In general, the complexity of contamination problems increases when NAPL’s are present. • Instead of one, there are up to three plumes to track: • vapour phase plume • immiscible liquid phase plume • dissolved phase plume

4. DNAPLs and LNAPL’s • NAPL’s are classified according to their density relative to water: • LNAPL’s (light non-aqueous phase liquids) are lighter than water and tend to “float” at the water-table and give rise to associated vapour phase and dissolved phase plumes. • DNAPL’s (dense non-aqueous phase liquids) are heavier than water and tend to “sink” to the bottom of aquifers where they can also give rise to dissolved phase plumes.

5. Mainly DNAPL Residual Mainly LNAPL Vapour Primary LNAPL Secondary LNAPL Primary DNAPL SecondaryDNAPL Complex Plumes Flow Clay Lenses

6. degree of localization point or local non-point or diffuse loading history pulse continuous contaminant type radionuclides trace metals nutrients other inorganics organics biological Contamination Attributes • Three important attributes distinguish sources of groundwater contamination:

7. Degree of Localization of Source • A point source is characterized by the presence of an identifiable small-scale source such as a leaking tank, a spill, a small pond or a landfill. • A diffuse source refers to a source emanating from many poorly defined locations. Pesticides, fertilizers, acid rain and highway salt are typical non-point diffuse sources.

8. Concentration Concentration Concentration Concentration Time Time Time Time Loading History • Loading history describes how the source concentration varies as a function of time. Pulse Continuous Constant Continuous Variable Continuous Decaying

9. A very long list of domestic, agricultural and industrial activities have the potential to result in contamination. Category I: Designed Discharges Septic tanks, injection wells, land application of wastes, etc Category II: Designed Storage Facilities landfills, dumps, tailings piles, ponds, underground tanks, etc Category III: Designed Transportation Systems pipelines, transport routes, transfer stations, etc Category V: Other Planned Discharges pesticide/fertilizer application, deicing salts, mine drainage, etc Category V: Potential Conduits production wells, monitoring wells, construction excavations, etc Category VI: Naturally Occurring Sources natural leaching, saltwater intrusion, saline water upconing, etc Sources of Contamination U.S. Office of Technology Assessment, 1984

10. Types of Contaminants • The list of potential contaminants number in the tens of thousands and organization of this list is a major problem. • Six major categories: (1) radionuclides (2) trace metals (3) nutrients (4) other inorganics (5) organics and (6) biological provide a framework for discussion. • All the contaminants have the potential to produce health problems. Too much of anything is a potential health hazard but tolerable thresholds are finite. • For some contaminants, particularly radionuclides, the threshold level is such that anything above natural background is of concern.

11. Radionuclides • The nuclear fuel industry is the main source of radioactive contaminants. • Potential sources occur throughout the nuclear fuel cycle. • During mining when raw ore is processed 238U, 229Th, 226Ra, and 222Rn are potential contaminants. • Fuel fabrication, fuel reprocessing and power-generating facilities are other potential sources. • Colorado, New Mexico, Utah, Wyoming, Saskatchewan and Ontario are the U producing regions of North America. • The nuclear industry is very closely controlled and monitored by federal, state and provincial authorities.

12. Trace Metals • Trace metals are a natural component of all groundwaters and represent the largest group of elements in the periodic table. • Additional sources include (1) mining effluents, (2) industrial wastewaters, (3) urban runoff, (4) agricultural wastes and fertilizers, and (5) fossil fuels. • Some trace metals (B, Cu, Fe, Zn) are essential for health but others have a tendency to accumulate in the body (or bioaccumulate in organisms low in the food chain) from sources at relatively low concentrations. • The 13 trace metals that make the EPA list of 129 priority pollutants are: Ag, As, Be, Cd, Cr, Cu, Hg, Ni, Pb, Sb, Se, Tl, Zn.

13. Nutrients • Nutrients are ions and compounds containing nitrogen and phosphorus. • The dominant nitrogen species in groundwater in NO3- and to a lesser extent NH4+. • Phosphorus is less important as a contaminant because of its low solubility and tendency to readily sorb onto solids. • Sources of N and P are largely agricultural including the use of fertilizers and the cultivation of virgin soils (when large quantities of N are released.) • Sewage (N) and municipal wastewaters (P) are also sources of nutrients.

14. Other Inorganic Species • This group of “contaminants” includes the major ions usually present in groundwater. • Extremely high concentrations render water unsuitable for human consumption, animal watering and many industrial uses. • Health related concerns are low for this group of contaminants but high concentrations of Na+ can disrupt blood chemistry and lower Na+ concentrations may lead to hypertension. • Fluoride a good example of a trace non-metal contaminant. At low concentrations F- has the beneficial effect of reducing tooth decay. At higher concentrations (only x5 higher) F- can lead to serious health problems including goitre and fluoridosis.

15. Organics • Contamination of groundwater by organic compounds is a consequence of the large number of petroleum products and man-made organics in common use. • The EPA list of 131 priority pollutants contains 116 organic compounds, 13 trace metals, asbestos and cyanide. Over 90% are organics. • The EPA divides the organics into four groups based on the methods used for analysis but ultimately the analytical tool is gas chromatography-mass spectrometry (GC-MS): • base-neutral extractables (47) • acid extractables (12) • volatiles (32) • pesticides and PCBs (25)

16. Petroleum Products • Organic compounds in crude oil and crude oil products fall into three main groups: • Alkanes (n-butane, n-hexane, cyclohexane, etc) • Alkenes (ethene, propene, etc) • Aromatics (BTEX group: benzene, toluene, ethylbenzene and xylene; PAH group: anthracenes, naphthalenes, etc) • Distillation separates crude oil into fractions: • Gasolines (C4 to C12 alkanes, C4 to C7 alkenes, BTEX group) • Middle Distillates (C10-C24 alkanes, some BTEX, some PAHs) • Residual Products (C20-C78 alkanes, aromatics mainly PAHs)

17. Halogenated Compounds • Aliphatics (chain molecules with H replaced by F, Cl, Br) including PCE (CCl2CCl2), TCE (C2HCl3) and CT (CCl4). • Aromatics (benzene rings with substituted halogens) including chlorobenzene (C6H5Cl) and dichlorobenzene (C6H4Cl2) • Both groups of halogenated compounds tend to have densities greater than water and usually fall in the DNAPL group.

18. PCBs • Polychlorinated biphenyls (PCBs) .Due to their non-flammability, chemical stability, high boiling point and electrical insulating properties, PCBs were used in hundreds of industrial applications prior to 1977 (when N.American production was discontinued). • They consist of joined, chlorine substituted benzene rings. • PCBs are relatively insoluble in water but are extremely persistent in the environment breaking down very slowly. • PCBs are highly toxic and highly soluble in biological lipids, and have been found to accumulate in animal and human tissues.

19. Organics in Groundwater water soluble DNAPLLNAPL

20. Biological Contaminants • The important biological contaminants are: • pathogenic bacteria (Fecal streptococci, Fecal colliforms, Escherichia coli, Shigella dysenteriae, Salmonella typhi, Vibrio cholerae) • viruses (enteroviruses, hepatitis A virus, polio virus and rotavirus) • parasites (Giardia, Entamoeba, Cryptosporidium ) • The main sources are (1) land disposal of sewage and septic tanks, (2) leachates from sanitary landfills and (3) agricultural wastes. • The contaminants are particulate rather than dissolved, transport distances are limited and problems tend to be related to localized sources.

21. Plume Morphology Basics • Mass transport processes control: • maximum plume extent • geometric distribution of concentrations • Advection is the dominant control on plume shape. • Hydrodynamic dispersion is usually a secondary factor. • Chemical, nuclear and biologic processes generally attenuate spreading.

22. Controls on Advection • Magnitude and direction of advective transport is controlled by: • hydraulic conductivity field • potentiometric head distribution • distribution of sources and sinks • shape of the flow domain • All these factors influence groundwater flow velocity, which drives advective transport.

23. H L Hydraulic Conductivity Field

24. Hydraulic Conductivity • Plume advects faster with increasing K. • Plume migrates in highest K horizon • Low K near surface delays plume. development. • Plumes tend not to invade low K units.

25. Dispersion aL=0 aT=0 aL>0 aL>>aT aL>0 aL>aT aL>>0 aL>aT

26. Dispersion • Increased dispersion gives broader plume. • Concentration pattern is spread and high concentration zone is reduced with increasing dispersivity. • Increasing aT results in increased vertical spreading. • High aL and high aT gives most mixing.

27. Reaction Rate Half-Life t1/2=30 t1/2=10 t1/2=3 t1/2=1

28. Reaction Rate Half-Life • Reaction removes contaminant mass. • As half life reduces, more material has reacted and the plume shrinks. • After about 5 half-lives the plume is indistinguishable from background. • Both biological degradation and radioactive decay result in plume shrinkage.

29. Sorption Kd=0.01 Kd=0.1 Kd=1 Kd=10

30. Sorption • Sorption is another attenuation process. • As Kd increases the plume becomes smaller as sorption retards and attenuates the mass in solution. • Sorption is a potent mechanism for reducing the extent of contaminant plumes.

31. Pulse Loading t =50 Co=1 Dt=5 Co=1/2 Dt=10 Co=1/4 Dt=20 Co=1/8 Dt=40

32. Pulse Loading • The same mass is added over an increasing time period from 5 to 40 years. • Peak concentrations are reduced by spreading and the entire mass is advected towards the discharge end of the flow system. • For lower source concentrations over longer periods the plume is closer to the source and less compact.

33. QA/QC in Water Sampling • Characterization of the distribution of contaminants in the subsurface is an integral part of any field study. • The collection of these data is fraught with difficulties and “bad data is worse than no data at all.” • Design of collection systems to ensure that data is both reliable and representative is a considerable challenge.

34. Screen Length • When piezometers are designed to measure hydraulic head, screen length is not a critical concern. • For chemical sampling, screen length can lead to widely variable results. 1 mg/L 10 mg/L 1 mg/L

35. Point Sampling • Point sampling implies a short screen section and small diameter access tube to minimize fluid volumes are avoid mixing and dilution. • Narrow tubes are not ideal for head measurements (where access for a tape is required). Chemical Monitoring Well Piezometer

36. Sampling Points • Sampling point locations need take account of the complexity of the flow system. • Plumes often have a restricted vertical extent and may not resolve a plume. • Multiport monitoring wells solve this problem (at a price). standpipe piezometers multilevel sampler

37. QA/QC for Chemical Data • There are four potential problems that can have a serious impact on chemical data: • Contamination of samples with drilling fluids. • Changes in water quality as a result of well construction. • Sample deterioration prior to analysis. • Careless field and laboratory practices.

38. Sample Contamination • Occasionally, samples become contaminated with gas, oil, or water in combination with foams, emulsions and muds used in the drilling process. • These contaminants are difficult to remove once the well has been completed. • Contamination can be avoided by using a readily detected tracer in the drilling fluid and developing the well until the tracer can no longer be detected

39. Well Construction • Cement and other soluble materials used in well construction can influence groundwater chemistry. • High pH (>9) is often characteristic of continuing interaction between cement and the water being sampled. • Samples can also be contaminated with trace metals from steel casing and adhesives from plastic casing joints. • Selection of threaded plastic casing avoids these problems. • Purging wells by removing at least 2 or 3 well volumes before sampling is also recommended.

40. Sample Deterioration • Temperature, pressure and gas content changes in samples following collection can seriously impact chemistry. • Exposure to atmospheric oxygen can lead to rapid Eh changes and metals precipitation from anoxic waters. • Loss of CO2 can raise pH and decrease bicarbonate concentrations (and perhaps precipitate CaCO3. • These problems can be addressed by specialized sampling equipment for high P-T environments, the use of flow-through cells and various sample treatments to minimize deterioration.

41. Field Sample Handling • Field handling of samples requires standardized bottle washing, filtering and use of chemically pure preservatives. • Sorption of organics onto containers or filters and loss of volatiles during samples are potential problem areas. • Running “blanks” of ultra-pure distilled water through the field sampling procedure allows for rapid detection of problems.

42. Laboratory Sample Handling • Ongoing QA/QC checks for analytical laboratories are a necessity. Common approaches include: • Submitting “spiked” samples of known composition for analysis. • Submitting duplicate samples to different laboratories. • Submitting replicate samples of the same water.

43. Sampling Systems • Sampling systems capable of providing point samples from the saturated zone include: • Nests of standpipe piezometers • Multi-level (or multiport) samplers installed in a single boreholes • Packers systems that can isolate various levels in a single borehole.

44. Cement Seal Fill Bentonite Seal Slotted Screen Sand Pack Nested Standpipes • Single wells are easier to complete and seal: • Drill • Sand pack • Bentonite seal • Backfill • Surface cement seal • Installation are durable and reliable. • Main disadvantage is high cost of drilling.

45. Multi-level Sampling • Multi-level samplers involve placing several sample points in a single hole: • seals are required between sample points • filter sand may or may not be required • sand packs are difficult to install • sand packs are often impossible to develop • small internal volume • reduced drilling costs

46. Multi-level Samplers • A wide variety of multi-level samplers are available: • multiple standpipes in single hole • bundled standpipes • valved suction couplings • retrievable downhole tools with packers and suction ports • gas-drive samplers

47. Bentonite Seal Sand pack Screen Multiple Standpipes Bundled

48. Suction Ports PVC Pipe Stainless Steel Screen Plastic Tube Rubber Seal Installation involves multiple ports and does not require sand pack or seals. Subject to clogging. Works best in cohesive materials.

49. Westbay-Type Samplers • Downhole sampling tool. • Casing has sampling ports • Packers inflated to isolate particular port • Suction recovers small sample • Transducers measure pore-pressure • Effective but expensive system

50. Unsaturated Zone Samplers • Most systems involve vacuum suction through porous ceramic cup. • Small sample chamber is filled. • Sample moved to the surface by suction or gas displacement. • Problems include: • reaction of pore fluids with ceramic • loss of gases as a result of vacuum suction