The PEI Government uses a rule of thumb that no more than 50% of our annual recharge can be taken back out. The 50% rule is so that we will still have lots of ground water left to feed our brooks and streams. This is a luxury that few places in the world can afford.
Over Pumping
In Long Island, New York in the 1930’s the water table was lowered to 30 feet below sea level. The natural path for water to travel is down hill, from the land to the ocean. However when the inland water levels are pulled down below sea level, the natural direction is reversed and sea water will enter the inland aquifers.
Miami and Los Angeles ran into similar problems with salt water pulled inland for up to 13 km.
These cities not only pumped more water than was being replaced by rainfall. But to make matters worse, they built storm sewers and canals to divert most of the rain water directly back to the ocean. Under natural conditions 30% to 50% of this rainfall would have entered the ground & recharged the aquifers.
Land Subsidence
The over pumping of ground water was so severe in Los Angeles that the land level fell by 3 feet. In Mexico city the land lowered 27 feet. This happens when thick claystone becomes dehydrated and shrinks, do to the over extraction of water from aquifers that lie beneath these claystone layers.
Land Uplift
In many areas Lakes are used for irrigation and Municipal water supply. When large bodies of surface water such as a lake are emptied the land can rise. Similar to the way PEI rose up after the Glaciers melted following the last ice age.
Here’s How It Works
Soft water falls as rain, filters down through the ground and eventually ends up back in the ocean. On its way the water picks up minerals from the bedrock, dissolves them, and carries the minerals to the ocean. The cycle continues, water vaporizes off the ocean surface leaving the minerals behind, and the ocean eventually turns salty.
What Happens When We Irrigate?
The well water put on the crops contains minerals. Rain has no minerals. Approximately 80 % or more of this water will evaporate either directly or through the transpiring of the plants. As the water evaporates the minerals (salts) are left behind.
This problem exists in parts of California and Arizona but has not been a problem in places with similar climates to PEI, for 2 reasons.
We irrigate for 3 months instead of 12.
We have a big recharge in the spring and fall that flushes the salts & minerals down from the surface.
How Are We Different?
On PEI, we do not have a climate that allows for more than one annual crop. Our average annual precipitation is 39 inches. ( 31 inches in 2001)
Many of the sedimentary rock types that lies beneath us can hold and transmit huge volumes of ground water, unlike the granites and metamorphic rocks in parts of Nova Scotia and New Brunswick.
We have a blanket of Clay (glacial till) that lies over our bedrock. This holds water long enough so that it can seep down into the bedrock and keep the aquifers below filled up.
The Big Water Users - 2003
The following is a list of some of the bigger water users on PEI. The numbers represent U.S. gallons (from wells only). The term return flow is the name given to water that is returned directly to the ground through septic tanks, or irrigation water that seeps back down in the ground.
City of Charlottetown: 5.1 Million Gallons Per day (3,500 per Minute)
City of Summerside: 1.6 Million Gallons per day (1,100 Per Minute)
Cavendish farms: 2 Million Gallons per day ? (Assumed use)
McCain food plant: 1 Million Gallons Per day ? (Assumed use)
All fish plants: Unknown
Food Plants like Humpty Dumpty: Unknown
Potato wash plants, Shellfish cleaners: Unknown
Blueberry & Cranberry operations: Unknown
Irrigation: Unknown
Return flow from irrigation = approx. 20% ( varies from 10 - 30 %)
Rural houses use approximately 200 gallons per day each (73,000 gallons annually). Most water used in rural houses is returned to the ground through septic tanks.
Rain Fall Facts
If one inch of rain falls over one square mile, the total would be 17. 3 million Gallons. That square mile would be equal to 640 acres of land. If one third of this water seeps down to become ground water, then 5.7 million gallons per square mile is available for every inch of rain.
Annual Rain Fall For Irrigation
Although PEI has averaged 39’’ of precipitation annually for many years, I will use the figures from 2001 because it was the driest year we had for some time. The precipitation for 2001 in Charlottetown was 31 inches.
The next step is to figure out whether or not there is enough ground water to irrigate a crop without pumping more water than is being recharged annually. For easy figuring we will use 640 acres (1 square mile).
Each time the farmer irrigates, he would aim to put on half an inch of water. It would be unusual to have to irrigate more than 12 times in one season (using a total of 6 inches of water.) Therefore the total water used to irrigate 640 acres would be 17.3 million Gallons x 6" = 104 Million Gallons. Less 20% return flow (21 Million Gallons) of water used to irrigate 640 acres of land that would end up being 83 Million Gallons.
If the same square mile (640 acres) received 31 inches of precipitation and if only one third ended up as ground water. The total recharge for that square mile would be:
17.3 X 31’’ = 536.3 million gallons
1/3 of 536.3 = 179 Million gallons that ends up as Ground Water. Subtract the 83 Million Gallons we used to irrigate, and you have 96 Million gallons of ground water left over.
In this worst case example we used 46% of the recharge to irrigate . 54% would be leftover to keep the rivers and brooks going. In reality, only one third of the land on that square mile can grow potatoes each year because of crop rotation.
Now the 46% is 15.3%. Any farm that has a percentage of woods and swamp land. Say 15% in a square mile is not farmed. The 640 acres becomes 544. Now the 15.3% becomes 13.3%.
Over Pumping
In Long Island, New York in the 1930’s the water table was lowered to 30 feet below sea level. The natural path for water to travel is down hill, from the land to the ocean. However when the inland water levels are pulled down below sea level, the natural direction is reversed and sea water will enter the inland aquifers.
Miami and Los Angeles ran into similar problems with salt water pulled inland for up to 13 km.
These cities not only pumped more water than was being replaced by rainfall. But to make matters worse, they built storm sewers and canals to divert most of the rain water directly back to the ocean. Under natural conditions 30% to 50% of this rainfall would have entered the ground & recharged the aquifers.
Land Subsidence
The over pumping of ground water was so severe in Los Angeles that the land level fell by 3 feet. In Mexico city the land lowered 27 feet. This happens when thick claystone becomes dehydrated and shrinks, do to the over extraction of water from aquifers that lie beneath these claystone layers.
Land Uplift
In many areas Lakes are used for irrigation and Municipal water supply. When large bodies of surface water such as a lake are emptied the land can rise. Similar to the way PEI rose up after the Glaciers melted following the last ice age.
Here’s How It Works
Soft water falls as rain, filters down through the ground and eventually ends up back in the ocean. On its way the water picks up minerals from the bedrock, dissolves them, and carries the minerals to the ocean. The cycle continues, water vaporizes off the ocean surface leaving the minerals behind, and the ocean eventually turns salty.
What Happens When We Irrigate?
The well water put on the crops contains minerals. Rain has no minerals. Approximately 80 % or more of this water will evaporate either directly or through the transpiring of the plants. As the water evaporates the minerals (salts) are left behind.
This problem exists in parts of California and Arizona but has not been a problem in places with similar climates to PEI, for 2 reasons.
We irrigate for 3 months instead of 12.
We have a big recharge in the spring and fall that flushes the salts & minerals down from the surface.
How Are We Different?
On PEI, we do not have a climate that allows for more than one annual crop. Our average annual precipitation is 39 inches. ( 31 inches in 2001)
Many of the sedimentary rock types that lies beneath us can hold and transmit huge volumes of ground water, unlike the granites and metamorphic rocks in parts of Nova Scotia and New Brunswick.
We have a blanket of Clay (glacial till) that lies over our bedrock. This holds water long enough so that it can seep down into the bedrock and keep the aquifers below filled up.
The Big Water Users - 2003
The following is a list of some of the bigger water users on PEI. The numbers represent U.S. gallons (from wells only). The term return flow is the name given to water that is returned directly to the ground through septic tanks, or irrigation water that seeps back down in the ground.
City of Charlottetown: 5.1 Million Gallons Per day (3,500 per Minute)
City of Summerside: 1.6 Million Gallons per day (1,100 Per Minute)
Cavendish farms: 2 Million Gallons per day ? (Assumed use)
McCain food plant: 1 Million Gallons Per day ? (Assumed use)
All fish plants: Unknown
Food Plants like Humpty Dumpty: Unknown
Potato wash plants, Shellfish cleaners: Unknown
Blueberry & Cranberry operations: Unknown
Irrigation: Unknown
Return flow from irrigation = approx. 20% ( varies from 10 - 30 %)
Rural houses use approximately 200 gallons per day each (73,000 gallons annually). Most water used in rural houses is returned to the ground through septic tanks.
Rain Fall Facts
If one inch of rain falls over one square mile, the total would be 17. 3 million Gallons. That square mile would be equal to 640 acres of land. If one third of this water seeps down to become ground water, then 5.7 million gallons per square mile is available for every inch of rain.
Annual Rain Fall For Irrigation
Although PEI has averaged 39’’ of precipitation annually for many years, I will use the figures from 2001 because it was the driest year we had for some time. The precipitation for 2001 in Charlottetown was 31 inches.
The next step is to figure out whether or not there is enough ground water to irrigate a crop without pumping more water than is being recharged annually. For easy figuring we will use 640 acres (1 square mile).
Each time the farmer irrigates, he would aim to put on half an inch of water. It would be unusual to have to irrigate more than 12 times in one season (using a total of 6 inches of water.) Therefore the total water used to irrigate 640 acres would be 17.3 million Gallons x 6" = 104 Million Gallons. Less 20% return flow (21 Million Gallons) of water used to irrigate 640 acres of land that would end up being 83 Million Gallons.
If the same square mile (640 acres) received 31 inches of precipitation and if only one third ended up as ground water. The total recharge for that square mile would be:
17.3 X 31’’ = 536.3 million gallons
1/3 of 536.3 = 179 Million gallons that ends up as Ground Water. Subtract the 83 Million Gallons we used to irrigate, and you have 96 Million gallons of ground water left over.
In this worst case example we used 46% of the recharge to irrigate . 54% would be leftover to keep the rivers and brooks going. In reality, only one third of the land on that square mile can grow potatoes each year because of crop rotation.
Now the 46% is 15.3%. Any farm that has a percentage of woods and swamp land. Say 15% in a square mile is not farmed. The 640 acres becomes 544. Now the 15.3% becomes 13.3%.