Groundwater Quality, Contamination, And Remediation

Groundwater Quality, Contamination, and Remediation

Overview • Groundwater may be the most important natural resource in the US • How do we define the quality of groundwater? • What are potential contamination sources? • The most productive aquifer in the world will be useless if the water is contaminated! • What can we do to prevent contamination OR remediate the contamination once it occurs?

Groundwater Quality • Groundwater almost always contains a higher dissolved mineral content than surface water • The quality of groundwater can be classified by the total dissolved solids (TDS) – How do you measure TDS? • Evaporate a known quantity of water and weight the remaining solids • Usually expressed in mg/L

– – – –

Fresh Brackish Saline Brine

<1000 TDS 1000-10,000 TDS 10,000-100,000 TDS >100,000 TDS

– Note - the chemical composition of the TDS are not considered in this classification scheme

Groundwater Quality • Where do TDS come from? – All minerals are soluble in water to some extent • May have extraordinarily low solubilities and slow dissolution rates

– Common soluble minerals • Any mineral that initially precipitated from aqueous solution – Salts, gypsum, calcite, dolomite

• Many minerals that act as cement in sedimentary rocks are fairly easily dissolved in circulating groundwater – Calcite, silica

• In all cases, usually a change in the chemical conditions is required for dissolution to occur (e.g., a batch of calcium carbonate undersaturated groundwater circulates through a limestone unit)

– Many TDS come from weathering reactions (hydrolysis, chemical weathering reactions)

Groundwater Quality • What components will be present? – Depends upon the path the water has taken from the initial recharge point to distribution within the aquifer – Water from limestone aquifer: • Will tend to have have calcium, magnesium, and bicarbonate concentrations

– Water from a gypsum-rich aquifer: • High sulfate levels

– Water from an iron-rich silica cemented sandstone aquifer: • High silica and iron concentrations • Fe > 0.3 mg/L will stain clothing and create solid precipitates

• Components – Major: > 5mg/L – Minor: 0.01 - 10 mg/L – Trace: < 0.1 mg/L

South Bend Groundwater • Northern Indiana has extremely good groundwater production (many wells exceeding 2000 gallons per minute - gpm) • Main aquifer source – Unconsolidated sand and gravel of Quaternary age (very young) – Locally > 450 feet thick – Three main units: • Unit 4 - sand and gravel lenses within a clay till (glacial debris) – High yields - as much as 3000 gpm – This unit supplies most of the water to South Bend

• Unit 3 - sand and gravel unit, partly artesian/partly water table – Again typically high yielding - as much as 3000 gpm – ~ 70% of groundwater pumped in St. Joseph county from Unit 3

• Unit 2 - silt till that acts as the confining layer for Unit 3

• Unit 4: – Underlies ~90% of St. Josephy county – As much as 350 feet thick in buried valleys – Deepest (non-bedrock) aquifer – TDS of ~400 mg/L (predominant component being CaCO3 and HCO3) – Fe 0.14 mg/L

• Unit 3 – Underlies whole county – Principle aquifer – 1/2 water table-1/2 confined – Variable TDS, recharge mostly from rain

South Bend Groundwater • Nearly all recharge comes from precipitation – Small amount from groundwater flow from Michigan and La Porte counties – During periods of drought and heavy pumping, small amount of recharge from St. Joseph river – Recharge to the artesian confined aquifer in Unit 3 occurs due to slow percolation through the overlying silt till

• Discharge – Natural discharge to rivers and streams and evapotranspiration in water table portion of Unit 3 aquifer – Wells in Unit 3 discharge ~32 mgd

• Quality – Bicarbonate, calcium, magnesium are major constituents – Locally, sulfate and iron can be significant

South Bend Groundwater • Units 3 and 4 have a potential yield from natural recharge of ~400 mgd • In 1969, 54 mgd were being drawn • Availability and long-term sustainability are not a problem • Main concern in this region is to keep the Unit 3 aquifer from being contaminated from agricultural products and waste

Groundwater and Construction • Engineers involved in the construction of dams, tunnels buildings, and highways spend much of their time trying to control or dispose of groundwater

Groundwater Contamination • Sources of Contamination – Table 12.1 and Figure 12.1

– Land surface contamination – Water table level contamination – Below water table contamination – Saltwater intrusion

Land Surface Contamination • Sources – – – – – – – – – –

Infiltration of contaminated surface water Land disposal of solid and liquid wastes Stockpiles, tailings, and spoil Dumps Salt spreading on roads Feedlots Fertilizers and pesticides Accidental spills Airborne particulate matter Composting of waste materials

• Point vs Non-Point sources

Land Surface Contamination • Surface water contamination has been a recognized problem throughout history • Remediation of surface drinking water began in mid 19th century and was common in industrialized countries by late 19th century • Link between surface water contamination and groundwater contamination was much later

Land Surface Contamination – A contaminated stream may, during usual conditions, be a discharge point for an aquifer – During a drought, or if a well is installed nearby, the lowering of the water table (or creation of ‘cone of depression’) can cause the stream to change to a recharge point and the contamination from the stream will enter the aquifer – Platt River atrazine contamination of wells (Lincoln, Nebraska) –Fig. 12.3

Contaminated Stream/Surface Water

Land Surface Contaminants • Disposal or stockpiling of wastes or materials is common practice • Liquid or solid wastes from sewage treatment plants, food processing companies, and other sources are applied to agricultural lands, golf courses, etc., to serve as fertilizers • Objective – allow chemical and biological processes in the soil, along with plant uptake, to break down the waste products into harmless substances

Land Surface Contaminants • This concept is generally successful, however, if any of the wastes are water soluble and mobile, these are typically carried into the subsurface • Hence, a problem arises if the fields are underlain by shallow aquifers • Example of water soluble and mobile contaminant – road salt!

APPLICATION OF PESTICIDES

UNCONTROLLED DUMPING

Land Surface Contaminants • Uncontrolled dumping of wastes often leads to leaching of contaminants and infiltration to the groundwater system seriousness depends upon the size of the dump and nature of the contaminants • Fertilizer and pesticide/herbicide applications – Most fertlizers are high in nitrate - nitrate is very soluble in water and easily can move into the groundwater system • many wells in agricultural areas have elevated nitrate levels • Elevated nitrate levels can cause blue baby syndrome

– Pesticides are usually less mobile but more toxic (wrt fertilizers)! • Mobility of pesticides usually controlled by the characteristics of the underlying soil and rocks • Karst regions with high hydraulic conductivity can have rather high pesticide concentrations in wells near agricultural areas

Land Surface Contaminants • Accidental Spills – Oil, gasoline, hazardous waste, liquid and solid human waste, chemical spills – These all can infiltrate aquifers and contaminate groundwater systems – Usually small in extent and can be cleaned up effectively if cleanup is initiated soon after spill

• Feedlots – Runoff from exposed feedlots can lead to high nitrate and phosphate levels in groundwater

Saltwater Intrusion • Coastal communities – Seawater intrusion is a serious problem in coastal areas – Near the shore there is an interface between fresh groundwater and saline groundwater from the ocean – Ghyben-Herzberg relation • Depth below sea level of the interface at any point inland from the shore is equal to 40 times the elevatation of the water table above sea level at that point – Water table elevation = 2 m, depth to interface is 80 m

Saltwater Intrusion • Coastal communities – Pumping lowers the water table in the vicinity of the well = rising of the interface between fresh and saline groundwater • Pumping lowers the water table 1 m, then the corresponding rise in the interface is 40 m • Wells can become contaminated by saline groundwater if overpumped

– Hard to recover from this type of contamination and many wells need to be abandoned