7.4 Groundwater quality & contamination

Natural groundwater quality

Groundwater chemistry reflects the minerals it contacts and its residence time. Total dissolved solids (TDS) is the sum of dissolved constituents in mg/L; fresh water is generally defined as below 1,000 mg/L TDS, brackish 1,000–10,000, saline 10,000–35,000, and brine above 35,000. The major ions dominate most analyses:

• Cations: calcium (Ca²⁺), magnesium (Mg²⁺), sodium (Na⁺), potassium (K⁺)
• Anions: bicarbonate (HCO₃⁻), sulfate (SO₄²⁻), chloride (Cl⁻)

A valid analysis should have a cation–anion charge balance within a few percent (milliequivalents of positive charge ≈ milliequivalents of negative charge). Water types are commonly displayed on a Piper (trilinear) diagram and evolve along the flow path — for example, from a Ca-HCO₃ type near recharge toward a Na-Cl type at depth.

Hardness is the concentration of divalent cations, mainly Ca²⁺ and Mg²⁺, expressed as mg/L of equivalent CaCO₃. Water below 60 mg/L is soft; 61–120 moderately hard; 121–180 hard; above 180 very hard. Hardness causes scale and consumes soap but is not a health hazard.

Worked example — hardness

Water contains 80 mg/L Ca²⁺ and 12 mg/L Mg²⁺. Convert each to CaCO₃ equivalent (multiply Ca²⁺ by 2.5 and Mg²⁺ by 4.1): Hardness = (80 × 2.5) + (12 × 4.1) = 200 + 49 = 249 mg/L as CaCO₃ → "very hard."

Contaminant transport

Dissolved contaminants move through aquifers by several coupled processes:

• Advection — transport with the bulk flow of groundwater at the average linear velocity v = Ki/n_e (see 7.2). This sets the travel time of the plume's center of mass.
• Dispersion — spreading of the plume relative to pure advection, from mechanical mixing (varied pore paths and velocities) plus molecular diffusion. Longitudinal dispersion (along flow) exceeds transverse dispersion; it dilutes peak concentrations and smears arrival over time.
• Sorption and retardation — partitioning of dissolved species onto aquifer solids (organic carbon, clays), which slows a solute relative to the water. The retardation factor R = v_water / v_contaminant ≥ 1:

R = 1 + (ρ_b · K_d) / n

where ρ_b is bulk density, K_d the distribution coefficient, and n porosity.

• Transformation — biodegradation, radioactive decay, and chemical reactions that reduce contaminant mass.

A plume is the three-dimensional body of contaminated water spreading downgradient from a source, elongated along the flow direction and shaped by advection, dispersion, and retardation.

Worked example — retardation

With bulk density ρ_b = 1.8 g/cm³, K_d = 0.25 cm³/g, and porosity n = 0.30: R = 1 + (1.8 × 0.25) / 0.30 = 1 + 0.45 / 0.30 = 1 + 1.5 = 2.5. If groundwater moves at 0.60 m/day, the contaminant advances at only 0.60 / 2.5 = 0.24 m/day.

Common contaminants

• Nitrate (NO₃⁻) — from fertilizer, septic systems, and manure; highly mobile and essentially non-sorbing; the drinking-water MCL is 10 mg/L as N; causes methemoglobinemia ("blue baby syndrome").
• VOCs — volatile organic compounds (benzene, TCE, PCE) from fuels, solvents, and degreasers; hazardous at microgram-per-liter levels.
• LNAPLs — light non-aqueous phase liquids (gasoline, diesel), less dense than water, so they float on the water table and spread laterally.
• DNAPLs — dense non-aqueous phase liquids (chlorinated solvents such as TCE and PCE, and creosote), denser than water, so they sink through the aquifer and pool on low-permeability bottoms, forming persistent secondary sources that are notoriously hard to remediate.
• Others: arsenic (often geogenic), petroleum hydrocarbons, pathogens, road salt, and per- and polyfluoroalkyl substances (PFAS).

Redox conditions and remediation

The fate of many contaminants depends on redox (oxidation–reduction) conditions, which evolve downgradient as microbes consume oxygen first, then nitrate, manganese, iron, and sulfate. Aerobic zones favor rapid degradation of petroleum hydrocarbons such as benzene, whereas chlorinated solvents like PCE and TCE dechlorinate only under strongly reducing (anaerobic) conditions. Common cleanup strategies include pump-and-treat (hydraulic containment plus extraction), permeable reactive barriers, in-situ chemical oxidation, bioremediation, and monitored natural attenuation (MNA), which relies on dispersion, sorption, and biodegradation to shrink a plume where site conditions allow. DNAPL source zones usually demand aggressive source removal because slow dissolution can sustain a plume for decades.

Saltwater intrusion

In coastal aquifers, fresh groundwater floats on denser seawater. The Ghyben–Herzberg relation approximates the depth of the freshwater–saltwater interface below sea level as:

z = [ρ_f / (ρ_s − ρ_f)] · h ≈ 40 · h

where h is the height of the water table above sea level. Thus every 1 m of freshwater head above sea level supports about 40 m of fresh water below it. Over-pumping lowers h, and the interface rises about 40 m for each 1 m of head lost — drawing saltwater toward wells (up-coning) and contaminating supply. Reduced pumping, injection barriers, and managed recharge counter intrusion.

Contaminant behavior summary

For the exam, pair each concept: advection with linear velocity, retardation with sorption and K_d, LNAPL with floating, DNAPL with sinking, and the Ghyben–Herzberg 40:1 ratio with saltwater intrusion.