Contamination of Groundwater

Groundwater is critically important to humans since it is a major source of drinking water.

Major threats to groundwater include spillage from underground storage tanks, effluent from septic tanks and leachate from agricultural activities, municipal landfills and abandoned hazardous waste sites (nitrates, volatile organic compounds, petroleum products, metals and synthetic organic chemicals.

Once contaminated, groundwater is difficult and expensive, in many cases impossible to rehabilitate.

SeawaterSeawater covers about 70 percent of the earth’s

surface. The average depth of the oceans is about 4 km.The saltiness of the sea is measured in terms of its

salinity, which is commonly expressed in parts per thousand (ppt). The salinity of average seawater is 35 ppt (equivalent 3.5%). The principal substances that contribute to the salinity of seawater are sodium and chloride ions.

Seawater is a source of:food and minerals

phosphate( an important fertilizer needed in agriculture and the land supplies are being depleted)

Manganese iron

coppercobalt Nickel

Seawater contains enough deuterium to make it a feasible source of energy.

The sea may provide new sources of food through aquaculture, the farming of the sea the way that the land is presently farmed. Some aquaculture projects have already started with the farming of oysters, clams and certain fishes.

Ocean and Seas

The body of salt water that covers the earth’s surface is called the ocean, or the sea.

The principal regions are:Atlantic Ocean- is second in size, with a surface area of 107 million km 2 and the shallowest average of 3.3 km.

Indian Ocean- has the smallest surface area , with 74 million, and an average depth of 3.8 km

Pacific Ocean- is the largest of the three principal ocean regions. It covers 180 million km2. It is circled by active converging plate boundaries, so it is sometimes described as being circled by a “rim of fire”.

Pacific Ocean

Indian Ocean

Atlantic Ocean

The Nature of Seawater

Sodium (table salt)Chlorine (sodium chloride)

Most abundant ions:

The concentration is increased by two factors:

Evaporation- increases the concentration because only water vapor evaporates, leaving the dissolved salts behind in greater concentration.

Formation of sea ice- ice that form from freezing seawater increases the concentration because when ice forms the salts are excluded from the crystal structure. Thus, sea ice is fresh water and the removal of this water leaves the dissolved salts behind in greater consumption.

The salinity of seawater is decreased by

Heavy precipitationmelting of ice

addition of fresh water by a large river

The ocean basin has not become filled in by the continuous supply of sediments and dissolved materials that would accumulate over millions years. The reasons for this are:

accumulated sediments have been recycled to the earth’s interior through plate tectonics

dissolved material are removed by natural processes just as fast as they are supplied by the rivers.

Movements of Seawater

Seawater is in constant state of motion, both on the surface and deep within.

The surface has two types of motion:

Waves, which have been produced some disturbance, such as the wind. Waves travel across the surface as a series of wrinkles that range form few centimeters high to more that 30m high. Waves crash on the shore as breakers and make the surf.

Currents, which move water from one place to another

Wave

Currents

WavesGentle wind produces ripples on the surface of

the ocean. Continuous wind produces larger and longer-lasting ripples. A ripple may eventually grow into an ocean wave, a moving disturbance that travels across the surface of the ocean.

Each wave has a ridge, or mound of water called a crest, which is followed by a depression called a trough.

Movement of the earth’s crust or underwater landslides can produce giant destructive waves called tsunamis.

The basic pattern of two high tides and two low tides every twenty-hours and fifty minutes can produce strong tidal currents in narrow bays.

crest

trough

Tsunami

Ocean Tides

Seawater moves in continuous ocean currents, streams of water that stays in about the same path as they move through other seawater over large distances.

Oceans currents are produced by:

1. density differences in seawater

2. winds that blow persistently in the same direction

Density Currents

The density of seawater is influenced by three factors:

the water temperaturesalinity

suspended sediments

The changes in the density of water result in the ocean current known as density current.

Surface CurrentsThese are broad and deep-running ocean currents

that slowly move tremendous volumes of water relatively near the surface.

The currents are modified by other factors such as the rotation of the earth and the shape of the ocean basins, but they are basically maintained by the wind systems.

Each ocean has a great system of moving water called gyre that is centered in the mid-latitudes.

The gyres rotate to the right in the Northern Hemisphere and to the left in the Southern Hemisphere because of the Coriolis Effect.

All are called “currents” except one that is called a “stream” (the Gulf Stream) and those that are called “drifts”.

The Ocean FloorThe landscape of the ocean floor remained largely a

mystery until the development of modern oceanographic research.

The regional landscape of the seafloor can now be studied with a variety of new instruments, including echo sounder, side scan sonar images, magnetometers and seismic reflection profiles.Seismic Reflection Profiles

The seismic reflection profile illustrates the type of information obtained about the seafloor by seismic reflection. The method involves emitting pulse of sound form a ship that is subsequently reflected back from the seafloor.

Ocean Floors

Seismic Reflection Profiles

The Oceanic RidgeThe oceanic ridge is the most pronounced

tectonic feature on earth. If the ridge were not covered with water, it would be visible from as far away as the moon. It is essentially a broad, fractured swell, generally more than 1500 km wide, with peaks rising as much as 3 km above the surrounding ocean floor. It covers nearly 23 percent of the earth’s surface, almost as much as the surface of the continents. It is the greatest “mountain” system on earth.

Mid-ocean Ridge

The Abyssal Floor

Vast areas of the deep ocean consist of broad relatively smooth surfaces known as the abyssal floor.

In most ocean basins, the abyssal floor can be subdivided into two sections:

abyssal hills- are relatively small hills, rising from 75 to 900m above the ocean floor. They are circular or elliptical and range from 1 to 8 km in width at the base. The abyssal hills can be considered as the most widespread landform on earth.

abyssal plains- are exceptionally flat areas of the ocean floor where the abyssal hills are completely buried by sediment. They are located near the margins of a continent, where turbidity currents transport sediment from a continental mass and spread over the adjacent ocean floor.

The Abyssal Floor

Trenches

A subduction zone, where two plates converge and one slab of lithosphere plunges down into the mantle, is generally expressed topographically by a trench.

A subduction zone is characterized by intense volcanic activity and seismicity. It is also marked by a large gravity anomaly. Trenches, some of which reach nearly 11,000 m below sea level, are the deepest parts of the ocean floor.

Trench

Japan Trench

Deep-Sea

Trench

Island and Seamounts

Thousands of submarine volcanoes occur on the ocean floor, with the greatest concentration in the western pacific. Some rises above sea level and form islands, but most remain submerged and are called seamounts.

They often occur in groups or chains, with individual volcanoes being as much as 100 km in diameter and 1000 m high.

Islands and Seamounts

Continental MarginsThe continental margins are covered by the ocean

but are not geologically part of the oceanic crust. They are composed of the continental crust and sediment derived from erosion of the land.

This part of the seafloor can be divided into three major sections:

continental shelf- is simply a submerged part of the shield or stable platform. It can be as much as 1500 km wide and its depth ranges from 20 to 550 m at its outer edge. At present, the continental shelves comprise 18% of the earth’s total continental area.

continental slope- descends from the outer edge of the continental shelf as a long, continuous slope to the deep-ocean basin. It marks the edge of continental granitic rock mass, the boundary between the continent and the ocean.

continental rise- is the transition between the continent and the ocean basin. It has a gentle, inclined surface, rising from the abyssal plains to the continental slope. It is apparently formed by sediment deposited at the base of the continental slope.

Submarine Canyons

Submarine canyons are common along the continental slope and have been studied for many years, long before the ocean floor was mapped. They have V-shaped profile and a system of tributaries; they closely resemble the great canyons cut by rivers on the continents.

Turbidity currents flowing from the continental shelf to the abyssal floor usually cut submarine canyons. Turbidity canyons can move at rates ranging up to 95 km per hour and carry a large sediment load capable of vigorous erosion.

Deep-Sea

Trench

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