BEIRUT ARAB UNIVERSITY FACULTY ACULTY OF ENGINEERING ENGINE ERING CIVIL & ENVIRONMENTAL ENGINEERING DEPT.
SANIT SANITARY ARY COURSE RSE (CIVE434)
CHAPTER 3: GROUNDWA GROUNDWATER TER FALL 2012-2013
Sources & Quality Groundwater is often superior in quality to surface water and generally less expensive to develop for use.
Sources & Quality • Overdraft occurs when groundwater is used faster than it is recharged. Most of aquifers are either in overdraft or are very close to being in overdraft. • Seawater intrusion happens when there is too much overdraft near the sea and saltwater seeps into the aquifer. •When saltwater gets into the aquifer, that part of the aquifer is contaminated and can never be used again. •Groundwater is the name used to describe water which is stored underground in permeable rocks. The period of storage may be a few weeks or several years depending on the geology.
Saltwater Intrusion Salt water is heavier than fresh water so at the margin between fresh and salt waters it will lie underneath the fresh water. The boundary between the two moves through the year, moving further out to sea when winter rainfall increases the pressure and volume of freshwater in the landward side of the aquifer. Where freshwater abstraction from the coastal aquifer is not in equilibrium with the flow, salt water can intrude into the zone of freshwater abstraction and reduce the quality of water supply.
Types of Water Wells Construction Classification
• Dug Wells: Are pumpless (water withdrawal by bucket) dug wells of varying degrees of formality. Such primitive dug wells were excavations with diameters large enough to accommodate men with shovels digging down to below the water table.
Types of Water Wells • Driven Wells: may be very simply created in clay soil or soft material with a "well point", which consists of a hardened drive point and a screen (perforated pipe). The point is simply hammered into the ground, usually with a tripod and "driver", with pipe sections added as needed. A driver is a weighted pipe that slides over the pipe being driven and is repeatedly dropped on it. When groundwater is encountered, the well is washed of sediment and a pump installed. • Drilled wells can get water from a much deeper level (more than 1000ms) by mechanical drilling. Drilled wells with electric pumps are currently used.
Types of Water Wells Hydraulic Classification
Normal or ordinary (shallow or unconfined) wells are completed in the uppermost saturated aquifer at that location (the upper unconfined aquifer). Artesian (deep or confined) wells, which are sunk through an impermeable stratum down into an aquifer which is sandwiched between two impermeable strata. The majority of confined aquifers are classified as artesian because the hydraulic head in a confined well is higher than the level of the top of the aquifer. Two additional broad classes of well types may be distinguished, based on the use of the well: Production or pumping wells, are large diameter (> 15 cm in diameter) cased (metal, plastic, or concrete). Monitoring wells or piezometers, are often smaller diameter wells used to monitor the hydraulic head or sample the groundwater for chemical constituents.
Surface Contamination of wells Is typically controlled by the use of a 'surface seal'. The well is typically cased from the surface down into the smaller hole with a casing that is the same diameter as that hole. The annular space between the large bore hole and the smaller casing is filled with bentonite clay, concrete, or other sealant material. Wells are typically capped with a well cap or seal that vents air through a screen into the well, but keeps insects, small animals, and unauthorized persons from accessing the well. At the bottom of wells, a screening device, filter pack, slotted casing, or open bore hole is left to allow the flow of water into the well.
Pollution of Groundwater Pollution of Groundwater may occur due to the following: -Water leachate from farms - Wastewater septic tanks - Solid waste landfill - Industrial production, petroleum and mining -Polluted wetlands and ponds
Pump Selection Each type of well pump has advantages and limitations. In working with your water well professional, review some of the following factors before making a final selection: • Adequate capacity (m3/min) for present and future use. • Adequate pressure for present and future use and for the possibility of a lower water level in the well. • Cost of the pump. • Cost of the labor to install the pump. • Cost of materials to install the pump, such as pipes, screen, fittings, accessories, well pit, etc. • Power supply. • Area needed to install the pump. Is there enough space available? • Reliability of the pump. • Cost and ease of servicing the pump. • Cost of operating the pump, including power and parts.
Q
K ( H
2
2
Ordinary well
h )
2 . 3 log( R / r )
Artesian well
Q
2 KB ( H
h)
2 . 3 log( R / r )
• A well in an unconfined aquifer is pumped at a rate of 25 lt/sec. The thickness of the aquifer is 15m and the elevation of the phreatic surface is 12.5m above the underlying aquiclude at an observation well 20m away from the well and 14.6m at a well 50m away. What is the value of K for this aquifer?
Q
K ( H
2
2
h )
2 . 3 log( R / r )
0.025 = 3.1415 K (14.6 2-12.52) /2.3log (50/20) K= 0.0001 m/sec
• A well in a confined aquifer with a thickness of 15m produces aflow of 25 lt/sec. The height of the phreatic surface is at an elevation of 114.6m at an observation well 50m away and at 112.5m at an observation well 20m away. • Find K for the aquifer.
Q
2 KB ( H
h)
2 . 3 log( R / r )
0.025 = 2*3.1415 K (15) (114.6-112.5) /2.3log (50/20) K= 0.0001 m/sec
Calcul ation o f Screen Length: Percentage of opening areas of screen is not more than 10% of total surface area of screen pipe. Total area of screen opening =0.10 d L L = length of perforated pipe d= diameter of perforated pipe V1 = entering velocity of water through the openings = 0.03 m/sec V2 = entering velocity of water through the pipe of the well=1.0 m/sec Q=(0.1 dL) V1 and V1 =0.030 m/sec Q=(0.1 dL) 0.03= 0.003dL ……………………..1 Q=A V2 = ( d2/4). V2 = ( d2/4)………………….2 Equation (1) = Equation (2) Q= 0.003dL = ( d2/4) ………………….L = 83.3d 90.00 d Example If d = 6 inch = 15 cm then L = 90 * 0.15 = 13.5 m ..take it = 16 m
Tests fo r Drinking Water from Private Wells Why sho uld I test my well? Municipalities are required to test their water supplies regularly to ensure the water is safe to drink. Since there is no requirement to test a private well except for bacteria when it is first drilled or the pump is changed, you are responsible for making sure your water is safe. Most private wells provide a clean, safe supply of water; however, contaminants can pollute private wells, and unfortunately you cannot see, smell or taste most of them.
What tests should be done on my water?
• Total Coliform bacteria • Nuisance bacteria • Nitrate • Pesticides
