Vident Partners provides vetted groundwater and hydrogeology expert witnesses for cases involving contaminant plumes, source identification, fate and transport modeling, Superfund cost allocation, natural attenuation, and groundwater discharges to surface water. Request a referral today.
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A groundwater expert decides where contamination came from, where it has gone, how fast it is moving and where it is headed. In most cases that means tying a plume to a source and a date, because liability, cleanup cost and exposure all depend on those answers. This page covers the movement of water and contaminants in the subsurface. How harmful a concentration is belongs to a toxicologist, the design of a cleanup system belongs to an environmental engineer, and PFAS chemistry and regulation have their own page.
The expert reads boring logs, well construction records, water level measurements, laboratory results over time, site histories, aerial photographs and models. A plume map is an interpretation drawn between wells, so the opposing expert will often ask what the data between those wells really show.
Groundwater flow rests on measured properties. Darcy's law expresses the factors controlling movement, including hydraulic conductivity, which depends on the size and arrangement of the pores and fractures 2. Travel time can vary enormously. The USGS describes shallow flow paths near streams with traveltimes of days to a few years, and the longest and deepest paths with traveltimes from decades to millennia 1. So a fixed average speed applied across a site is an easy target on cross-examination.
Models carry that work into the future or the past. MODFLOW, the modular hydrologic model from the USGS, is considered an international standard for simulating groundwater conditions 3. The name does not settle anything. A model is only as good as its inputs, and the fights are about boundary conditions, calibration and whether a different set of assumptions fits the same data. Age dating a release by model is especially exposed, because the answer inherits every uncertainty in the inputs.
Chemicals heavier than water add a further problem. EPA explains that the presence of a dense nonaqueous phase liquid can be inferred where a dissolved concentration exceeds 1% of the chemical's pure-phase solubility, a rule of thumb it calls very conservative because thin, highly concentrated plume cores can be diluted by conventional well construction and sampling 4. That means a monitoring well result may understate what sits nearby, which matters to both source and cost.
Superfund remedies start from a stated expectation: EPA expects to return usable ground waters to their beneficial uses wherever practicable, within a reasonable timeframe, and where that is not practicable, to prevent further migration of the plume and prevent exposure 6. Much of the argument is about what is practicable and how long is reasonable.
Monitored natural attenuation is a frequent dispute, because one side sees a remedy and the other sees no action. For inorganic contaminants, EPA's directive sets a phased approach that begins with a demonstration that the plume is not expanding, then asks whether the mechanism and rate of attenuation are sufficient and whether the aquifer has capacity 5. Each phase is a place for a hydrogeologist to test the record, and organic chemicals have their own guidance.
In contribution suits, a court may allocate response costs among liable parties using equitable factors it finds appropriate 8. A hydrogeologist often supplies the factual base: which release reached which part of the plume, and in what share.
Groundwater also carries Clean Water Act exposure. In County of Maui v. Hawaii Wildlife Fund, the Supreme Court held that a permit is required for a direct discharge or for the functional equivalent of a direct discharge 7. It said time and distance will be the most important factors in most cases, and that other factors may include the material the pollutant travels through and the extent to which it is diluted or chemically changed 7. Those are hydrogeology questions, and the answer is specific to the site.
Look for a hydrogeologist with a state professional geologist license, or a professional engineer with groundwater experience, who has worked in the same type of aquifer. Experience with fractured rock does not automatically transfer to a sand aquifer. Ask for direct field and modeling work on comparable contaminants. An environmental engineer designs the remedy, a toxicologist addresses health risk, and a forensic chemist can help identify a source by chemical fingerprint.
In County of Maui, the Supreme Court said time and distance will be the most important factors in deciding whether a discharge through groundwater is the functional equivalent of a direct discharge.
Case Types
Contaminant source identification and plume age disputes
CERCLA cost recovery and contribution allocation
Leaking underground storage tank and solvent release claims
Drinking water well contamination claims
Clean Water Act citizen suits over discharges through groundwater
Disputes over monitored natural attenuation and remedy selection
Groundwater withdrawal and well interference claims
Property damage claims from migrating contamination
Qualifications
Related Specialties
FAQ
Look for a licensed professional geologist, or a professional engineer with real groundwater experience, who has worked in the same type of aquifer with comparable contaminants. Field work with wells and aquifer tests matters as much as modeling skill, and experience in one geologic setting does not automatically transfer to another.
Sometimes, within limits. An expert can use travel time, chemistry and site history to estimate a window. Travel times vary widely with the flow path, and a modeled date inherits every uncertainty in its inputs. A credible opinion states the range and the assumptions rather than a single date.
MODFLOW is the USGS modular hydrologic model and is considered an international standard for simulating groundwater conditions. Using it does not by itself make an opinion reliable. The dispute is usually about boundary conditions, calibration and whether other assumptions fit the same data.
It relies on natural processes to reduce contamination, with monitoring to confirm it is working. For inorganic contaminants, EPA's directive begins with showing that the plume is not expanding, then tests whether the mechanism, rate and aquifer capacity are sufficient. Organic chemicals have separate guidance.
Under County of Maui, a permit is required when a discharge through groundwater is the functional equivalent of a direct discharge to navigable waters. The Supreme Court said time and distance will be the most important factors in most cases. Other factors include the material the pollutant travels through and how far it is diluted or changed.
In general, groundwater expert fees are determined by the expert themselves, based on a variety of criteria. Among those criteria are licensure, field experience, modeling experience and the volume of site data to review. Vident does have some influence over expert fees by comparing experts within a specialty, but ultimately it is a personal decision by the expert.
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