GY 111 Lecture Notes D Haywick 2008 09 1 GY 111 Lecture Note Series Groundwater and Hydrogeology Lecture Goals A The hydrologic cycle B Groundwater dynamics C Mapping groundwater done in class not on the web notes A The hydrologic cycle At the time I am writing these notes early October 2008 the economy of the world is in major trouble Banks are failing money is drying up and even profitable geological ventures like oil gold and base metal exploration are starting to lose their appeal No mention of job losses in these geology professions yet but if no one including businesses has any capitol to buy stuff it will be difficult for geological companies to sell copper or zinc or even oil But you know there is one other commodity that geologists explore for that is unlikely to be impacted by any economic slowdown It s just too important water Water is vital for most life forms on the planet and there is a lot of it It is one of the most mobile substances on the planet and it is constantly being recycled from one form to another through the hydrologic or hydrological cycle The hydrological cycle is a bit like the rock cycle but instead of relating rocks and processes to one another it relates water and processes It is defined as all of the water on over and in the Earth and the processes by which that water moves between reservoirs As they say a picture is worth 1000 words and the picture below saves me a lot of letters The hydrologic cycle consists of 5 main reservoirs storage places for water In order of descending volume they are 1 Oceans 98 of all water 2 Ice caps 1 8 3 Groundwater 0 6 4 Lakes and rivers 0 01 5 Atmosphere 0 001 GY 111 Lecture Notes D Haywick 2008 09 2 Most people understand that the oceans and ice caps contain most of the planet s water but they are usually surprised that groundwater not river lake water is the 3rd most important reservoir in the hydrologic cycle Truth be told groundwater contains 40x as much water as all the rivers and lakes on the surface of the Earth which is the reason why it is so important to much of the world s population including the good old US of A Anyone that has ever walked in Mobile in the summer knows that the volume of the reservoirs is not fixed i e summer humidity in Mobile is extreme and it sometimes feels like there is more water in our atmosphere than in the oceans Water is recycled between each of the reservoirs via the processes of 1 evaporation water to gas 2 condensation gas to liquid e g clouds 3 runoff liquid water flowing over the land e g rivers 4 infiltration liquid water flows into the ground groundwater The cartoon above shows the hydrologic cycle budget A staggering 100 million billion gallons of water a year are cycled through all of the reservoirs 1 1000th of the total water on the planet Note that the majority of evaporation and condensation occurs over the oceans especially in the tropics Only 3 of the condensation falls on land where is joins moisture evaporated from lakes rivers and wet soil Rivers are the products of runoff which is defined as water flowing on the surface of the Earth Infiltration occurs when water sinks into the soil to become groundwater It too can also flow back toward the sea but it is much much slower than river water Groundwater is one of the most important resources that we have In many parts of the world including a surprisingly large percentage of the USA groundwater is the only viable supply of water e g Baldwin County In the United States 50 of Americans depend on groundwater for their drinking water more if you include all GY 111 Lecture Notes D Haywick 2008 09 3 those bottled water thingies we drink Forty percent of irrigation water and 26 or industrysupplied water comes from underground And these numbers are increasing daily1 So how does groundwater get into the ground The first thing you need is water rainwater river water lake water etc Then you need surface materials that are both porous i e have lots of void spaces between grains and permeable i e all of the void spaces are connected Well sorted beach sand is an excellent porous material but broken up fractured and mechanically weathered bedrock will all allow infiltration Water that enters the voids is called pore water and it tends to move downward The mechanism driving infiltration is gravity and it s the same reason that water flows downhill in rivers So as water infiltrates the subsurface it is moving downhill that is straight down The actual path that the water takes is tortuous because it has to flow through the pores between grains see previous cartoon and it is a relatively slow process But if the material at the surface of the Earth is both porous and permeable eventually a lot of the water that falls during rain storms will infiltrate the soil If all of the pore spaces get filled up with water the excess runs off into streams and rivers Water that passes into the ground will continue to percolate downward until it hits a barrier e g a non porous layer or the bedrock or it simply fills up all of the available pore space You can actually identify two separate layers or horizons below the surface of the Earth that are distinguished by the amount of water that fills pore spaces The layer where water only partially fills pore space is called the Zone of Aeration or vadose zone and the layer below this where water entirely fills pore space is called the Zone of Saturation or phreatic zone The two layers are separated by an important plane called the water table More on this shortly but first Let s reconsider the whole infiltration runoff concept a bit more particularly as it applies to rivers and lakes Some lakes and rivers particularly in arid and desert areas lose considerable amounts of water to infiltration Some eventually dry up entirely These rivers and lakes are said to be influent Others actually receive a lot of water from groundwater draining into them this happens a lot in our area and explains why rivers still have flowing water in the middle of prolonged droughts These rivers and lakes are said to be effluent adjacent diagram from http www kgs ku edu 1 The data I gave you here is really old 1970 s and I can assure you all of these numbers are up considerably for 2007 08 GY 111 Lecture Notes D Haywick 2008 09 4 Back to the concept of the water table separating the zones of aeration and saturation In order to actually get water out of the ground it is necessary to penetrate below the water table This