Groundwater Lecture Notes PDF

Contaminated Sites Instructor: Dennis Klick Class 5 Agenda Topics to be covered Groundwater Principles Geological Concepts Geological Cross-Sectional Profile Groundwater Principles Over 70% of the earth’s surface is covered in water Only 1% of earth’s water is readily available for human use 90% of...

Contaminated Sites Instructor: Dennis Klick Class 5 Agenda Topics to be covered Groundwater Principles Geological Concepts Geological Cross-Sectional Profile Groundwater Principles Over 70% of the earth’s surface is covered in water Only 1% of earth’s water is readily available for human use 90% of the earth’s supply of freshwater is stored in underground reservoirs GW is the water found underground in the cracks and spaces in soil, sand & rock The area where water fills these spaces is called the saturated zone GW is stored in and moves slowly through geologic formations of soil, sand and rocks called aquifers Groundwater Principles The top of the saturated zone (called the water table) may be only a foot below the ground’s surface, or it may be hundreds of feet below The water table may be shallow or deep; and may rise or fall depending on many factors Groundwater is unevenly distributed underground in both quality & quantity Groundwater doesn’t flow like an underground river Typically composed of gravel, sand, sandstone, or fractured rock, aquifers allow large underground reservoirs of water to accumulate Groundwater (GW) GW may be easily polluted by toxic chemicals & other hazardous materials The most common mode of contaminant migration in the subsurface is advective flow with groundwater “Water table” describes the top of the zone of saturation The process by which groundwater exits the ground is known as groundwater discharge This groundwater can either discharge directly into oceans, or more commonly, it discharges to surface water (lakes and rivers) & travels to the ocean as surface runoff Porosity Porosity is a measure of the void spaces in a material The greater number of pore spaces, the higher the porosity thus mire GW it can hold Porosity is largely influenced by particles size, shape, assortment and packing Permeability Permeability refers to the rate (measure) at which water moves through the soil - also known as Hydraulic conductivity Permeability is the capacity of a material to transmit fluids, which is mainly controlled by the size of the pore spaces and the degree to which they are interconnected Coarse-textured sandy & gravel soils have the largest pores & most rapid permeability Dense, compact or cemented soil layers have very slow rates of permeability Permeability rates are given in inches per hour. Porosity vs Permeability Porosity and permeability are related properties of any rock or loose sediment Both are related to the number, size, and connections of openings in the rock Specifically, porosity of a rock is a measure of its ability to hold a fluid - amount of pore space that is between particles in soil or rocks Permeability is a measure of the ease of flow of a fluid through a porous solid -takes this pore space and connects the voids together so that water can pass through. Porosity is more associated with storage of water, while permeability is more associated with groundwater movement and flow Typical Porosity & Permeability Ranges Typical permeability rates: 0.01’’ per hour for compact clay 0.5’’ per hour for loamy soil 15” per hour for sand Soil Texture Soil texture refers to the proportions of sand, silt and clay in soil A ‘loam’ refers to balanced-mixture of sand, silt and clay Unbalanced mixtures dominated by increasing amounts of sand are called sandy loam, loamy sand and just plain sand If clay dominates, the texture is called clay-loam Silty soils that contain little or no sand are called silty-clays Soil Texture Chart Soil texture triangle are scaled for the percentages of sand, silt, and clay Clay percentages are read from left to right across the triangle Silt is read from the upper right to lower left Sand is read from lower right towards the upper left portion of the triangle The boundaries of the soil texture classes are highlighted with a bolder line The intersection of the three sizes on the triangle gives the texture class Classify a soil sample that is 30% clay, 15% silt, and 55% sand Classify a soil sample that is 60% clay, 20% silt, 20% sand Classify a soil sample that is 15% clay, 40% silt, 45% sand Classify a soil sample that is 30% clay, 60% silt, 10% sand Groundwater Industrial pollutants can be found in GW Common dissolved constituents in groundwater include: Ions of sodium calcium magnesium iron chloride bicarbonate sulfate Many other substances are dissolved in groundwater at lower concentrations GW Contamination Sources Storage Tanks Application of agricultural herbicides and pesticides Seepage from landfills Mines Septic Systems Uncontrolled Hazardous Waste Landfills Chemicals - Road Salts Atmospheric Contaminants Point - Source Pollution Any contaminant that enters the environment from an easily identified and confined place Comes from a single place From an environmental perspective, it is usually easier to deal with point source emitters than non- point Examples: Smokestacks Discharge pipes Drainage ditches Municipal wastewater treatment Non-Point Source Pollution Non-point sources comes from many diffuse sources May occur over thousands of square kilometres Nonpoint source pollution is the leading remaining cause of water quality problems Nonpoint source pollutants on specific waters vary & may not always be fully assessed Examples: Excess fertilizers, herbicides and insecticides from Agriculture Oil, grease and toxic chemicals from Urban Runoff Road salt GW Flow Concepts Important concepts for upcoming assignment GW flows from higher hydraulic head to lower head (represent the energy of stored a fluid) GW flows generally from upland areas to lowlands - from hills slopes to streams & surface water bodies Some paths are short & local, while others go deeper, before discharging GW flows can be long regional journeys to the lowest drainage in the area GW can flow a few cm to a few m a day in sand or gravel aquifers GW can also move several m/day or more in some highly fractured bedrock aquifers (fast movement) Aquifers Groundwater is stored underground in the pore spaces of saturated soil & rock materials An underground unit of soil or rock through which large volumes of water can flow & be stored is called an aquifer Groundwater flows through interconnected pore spaces in aquifers Groundwater may flow at different rates in different types of aquifers Confined vs Unconfined Aquifers Unconfined aquifers are those into which water seeps from the ground surface directly above the aquifer Unconfined aquifers are free of impervious layers like clays and shales Water seeps from the ground surface directly above the aquifer Confined aquifers are bound by above and below by impermeable layers Layers prevents water from seeping into the aquifer from the ground surface located directly above Confined aquifers also known as artesian aquifers Geological Layers - Aquitards An aquitard is also a saturated formation layer It permits the water through it but does not yield water in sufficient quantity as much as aquifer does - partly permeable If there is an aquifer under the aquitard then the water from aquitard may seep into the aquifer Example of an aquitard: Sandy clay Geological Layers - Geological Layers- Aquiclude An aquiclude is a geological formation which is impermeable to the flow of water. Contains a large amount of water in it but it does not permit water through it and also does not yield water. It has high porosity Example of aquiclude: Clay is an Geological Layers - Aquifuge An aquifuge is an impermeable geological formation which is neither porous nor permeable cannot store water in it &at the same time it cannot permit water through it Compact rock is an example of aquifuge Confined vs Unconfined Aquifers Geological Layers Overview Geological formations/ Properties Aquifer Aquitard Aquiclude Aquifuge Water storage Yes Yes Yes No Permeability of water Permeable Partly permeable Impermeable Impermeable Yield of water Yes Yes but slow yielding Do not yield Do not yield Clay Compact rocks such as granite, basalt etc. Examples Sand, Gravel Sandy clay GW Recharge GW is recharged naturally by rain and snow melt and to a smaller extent by surface water (rivers and lakes) - causes the water table level to rise Such a rise of water table is referred to as recharge Recharge may be impeded somewhat by human activities including paving, development, or logging. Takes time for surface infiltration to reach an aquifer as deep as 400 feet may take hours, days, or even years, depending on the rate of recharge Artificial groundwater recharge is becoming increasingly where over-pumping of groundwater has led to underground resources becoming depleted Groundwater Challenges Governance of groundwater so challenges include: The effects of groundwater mismanagement are not observed until long after they occur Once the problem is recognized the remedy requires a long period of time although groundwater problems have common aspects throughout the world, each problem is local in nature requiring site specific solutions Drinking Water Wells Wells are drilled into aquifers to recover GW When a well is pumped, a cone of depression - a local gradient in the water table toward the well is formed The distance the water table decreases with the pumping is called the drawdown Size & shape of the ‘cone’ depends on factors such as: material & thickness of the aquifer amount of water in the storage pumping rate Cone of Depression Cone of Depression is formed where the water table sinks at an exact point because of heavy pumping Water is extracted out of the ground which leaves an airy space around the soil particles If the pumping rate is faster than that of water that is flowing in, it results in the drawdown of the water table at the location of the pump Over-Pumping GW If GW pumping exceeds recharge water levels will not equilibrate and eventually the aquifer is depleted. Often results in the disappearance of stream flow and subsidence of the land surface As a result, the water table below the stream or lake gets reduced, and water starts moving towards the groundwater aquifer near the well GW Over-Pumping Concerns Pumping water out of the ground faster than it is replenished over the long-term causes similar problems Some of the negative effects of groundwater depletion: drying up of wells reduction of water in streams and lakes deterioration of water quality increased pumping costs land subsidence Over-Pumping Cone of depression can prove to be useful when dealing with contamination of groundwater. A well can be developed near the contaminated area, and then, pumped at a sufficient rate to create a cone of depression It then, captures the contaminated flow which can be treated to dispose off the contaminants and render clean water. Cone of Depression in Unconfined Aquifer The area around a well in an unconfined aquifer is normally saturated, but becomes unsaturated when the water is pumped through a well. The water table then dips down to form a cone of depression Three wells in an unconfined aquifer. Well A is not being pumped. Well B is being pumped at a slow rate Well C, which has a larger cone of depression, is being pumped at a faster rate Induced recharge Cone of depression can stretch to the nearby water bodies such as lakes or streams This is known as ‘induced recharge’ Geological Cross- Sectional Profile Geological Cross-Section Geologic cross sections provided two-dimensional slice of Earth's subsurface Used to help understand geologic conditions that occur in specific areas of the cross section Geologic maps, core, well cuttings, geophysical logs, and driller’s logs provide valuable geologic controls for cross sections Adding hydrogeologic data (i.e. water levels) to cross sections can help provide insights into the groundwater (aquifer) systems Constructing Geologic Cross Section Profile Decide what the geologic cross section is going to be used for & use this to guide in selecting the appropriate scales Choose appropriate vertical and horizontal scales If the cross section is to have the same horizontal scale as the map, the scales of the map and cross section are the same To enlarge cross section order to see additional detail, reduce the ratio of the scales Example: If the map scale is 1 inch equals 40 feet, making it larger would require changing the horizontal scale on the cross section to 1 inch equals 20 feet Constructing Geologic Cross Section Profile On the map, locate the well or borehole positions, land service elevations, depth of the well and the number of geologic units in each well bore Transfer the geologic information from each will long to the cross-section This information represents discrete points of knowledge about the subsurface geology Part of the geologist skills is interpretation from these discrete points of knowledge to those areas that lie in between Constructing Geologic Cross Section Profile Correlate the geologic information between boreholes Applying knowledge of the specific depositional features of the rock or sediments can be used to increase the accuracy of the model. Look for differences in lithology, texture, or sediment or rock properties as a guide to defining contacts between contiguous geologic units. Use solid lines to indicate reasonably certain relations between discrete data points. Dashed lines are used to indicate uncertainty or inferred data. Areas where does not exist are typically labeled with question marks Constructing Geologic Cross Section Profile Incorporate a legend into the cross-section to explain the types of geologic materials present. The legend should be placed at the bottom of the profile Use appropriate orientations and landmark information to help the viewer relate to the cross sections position in space relative to recognizable features (buildings, streets, streams, etc.) Include vertical and horizontal scales along with the statement of vertical exaggeration BC Groundwater Protection Regulation BC Groundwater Protection Regulation Establishes standards to protect ground water supplies by requiring all water wells in BC to be properly constructed, maintained, and, at the end of their service, properly deactivated and ultimately closed Groundwater wells provide industry, municipalities, farms and homeowners with access to groundwater to meet domestic & non-domestic water needs Groundwater wells must be constructed and maintained to ensure that the groundwater supply is safe from pollution, and the possibility of wasted water is minimized British Columbia – Groundwater Well & Aquifer Database BC Groundwater Protection Regulation Surface Seal – to prevent contaminants from the surface or a shallow subsurface zone from entering the well. Seal must be at least 2.5 cm. (1-inch) thick Secure Well Cap – to prevent direct and unintended entry into the well of any water or undesirable substances at the surface of the ground, including floodwater, ponded water, and contaminants Well Casing Stick-up – to help floodproof the well. Stick-up must be at least 30 cm. (12 inches) above ground surface or the floor of pump house to the top of the casing BC Groundwater Protection Regulation Wellhead Graded – to drain surface water away from the wellhead Well Identification (ID) Plate – well drillers are responsible for attaching a well identification plate to a new water supply well Controlled or Stopped Artesian Flow – to prevent wasting water, the driller must construct the well in a manner that stops or controls any artesian flow Deactivate a well Wells that have not been used for 5 years must be deactivated Deactivating a well means capping, securing, protecting, and maintaining the well in a safe and sanitary condition while it is out of service Deactivated wells not used for 10 years must be properly closed Closure involves backfilling and sealing the well Groundwater Wells Dug Well Dug (or excavated) wells are typically only 10 to 30 feet deep Being shallow - dug wells have the highest risk of becoming contaminated An improperly capped well is more vulnerable to contamination and presents a safety hazard The BC Groundwater Protection Regulation requires all wells be fitted with secure, verminproof well caps or covers to prevent the direct and unintended entry of contaminants, persons or animals into the well Drilled Wells Well built with machines for rotary or percussion drilling. There are drilled wells that go deeper than dug wells, penetrating unconsolidated materials which can affect the quality of the water Drilled wells can be more than 1000 feet deep An excavation lined with wood, stone or metal cribbing Places wellhead below ground to protect from freezing Drilled Wells in Pits Drilled wells located within a pit Drilled wells can be more than 1000 feet deep Pits can become infested and are commonly flooded. (Risks: debris, bacteria, pesticides, fertilizers, disease) Pits are a health hazard to enter (Risks: low oxygen, high levels of carbon dioxide, other gases) Well Contamination Sources BC Groundwater Regulation requires drinking water wells to be a minimum of 30 meters or 100 feet from potential contaminant sources: Pesticides Vehicles Fertilizers Fuel Animals Septic Tanks Storage Tanks Waste