Groundwater

Lecture  notes       Groundwater     GLG  101  –  Physical  Geology   In  the  hydrologic  cycle,  what  happens  to  the  water  that  infiltrates  into  the  ground?    It  becomes   groundwater.      Fresh  water  amounts  to  less  than  3%  of  the  water  on  Earth,  and  groundwater  is  only   0.62%  of  the  water  on  Earth.    Although  it  is  a  relatively  small  amount,  in  the  United  States,  21%  of  the   fresh  water  we  use  comes  from  groundwater,  and  in  Arizona  50%  of  our  fresh  water  comes  from   groundwater.  That’s  why  it  is  so  important,  particularly  in  the  dry/arid  southwest.   What  happens  to  water  precipitating  on  ground  surface?   • Over  50%  evaporates  into  atmosphere   • Large  amount  also  transpires  from  vegetation   • Water  infiltrates  into  the  ground  from  streams  (15-­‐20%)  and  precipitation  on  Earth’s  surface,   ending  up  as  groundwater  –  water  flowing  underground   • Water  that  does  not  infiltrate  from  streams  ends  up  as  runoff  to  lakes/oceans   These  are  some  of  the  main  factors  that  influence  how  much  water  can  get  into  the  ground   • Slope  –  steepness   • Amount  of  precipitation  –  rain/snow   • Amount  of  vegetation   • Surface  material  –  must  be  porous  &  permeable   Porosity  &  Permeability  are  important  factors  in  groundwater  movement  and  storage   Porosity  =  The  amount  of  empty  space  in  a  rock  measured  in  percent  (%)   • Unconsolidated  material=  sediment  or  soil  (e.g.  gravel,  sand  and  clay)  -­‐  porous  25-­‐80%   • Clastic  rocks  are  generally  less  porous  (compaction/cementation).    These  are  some  average   porosities  by  rock  type.     • conglomerates  &  sandstones  (10  to  30%)   • mudstones  (0  to  30%)   • Crystalline  rocks  (igneous/meta/chem  sed)  =  low  porosity-­‐increases  with  fractures   Permeability-­‐  the  capacity/ability  of  a  rock  to  transmit  a  fluid  (allow  the  flow  of  water  within  it).   • Pores  or  fractures  are  connected  with  each  other.     • Rock  must  be  permeable  to  allow  the  infiltration  and  flow  of  water.           • Unconsolidated  sands/gravels  –  These  are  very  permeable   • Clastic  sandstones/conglomerates  –  These  are  permeable  to  moderately  permeable   • Clay,  mudstones,  and  unfractured  crystalline  rocks  –  These  are  not  very  permeable.       • Fractured  rocks  -­‐  These  may  be  permeable  if  the  fracture  systems  are  connected.   Important  note:    A  high  porosity  does  not  always  mean  there  is  a  high  permeability.    For  example,   clay=low  permeability  and  high  porosity,  and  sand=higher  permeability  and  higher  porosity.   Aquifer-­‐Aquitard-­‐Aquiclude.    We  define  the  rock  units  by  their  ability  to  retain  or  transmit  water.     Aquifer  -­‐-­‐  permeable  rock  strata  or  sediment  that  transmits  groundwater  freely  (e.g.   sands  and  gravels  or  fractured  limestone).    An  aquifer  is  permeable  enough  to  allow  for   the  storage,  flow  and  pumping  (extraction)  of  groundwater.   Aquitard  -­‐-­‐  an  impermeable  layer  that  hinders  or  prevents  water  movement  (e.g.  clay)   Aquiclude  -­‐-­‐  not  porous  or  permeable  and  prohibits  the  passage  of  groundwater  (e.g.   unfractured  crystalline  rocks)   Now  that  we  know  what  the  main  rock  units  are  called,  we  can  look  at  the  “zones”  beneath  the   surface  that  form  when  water  infiltrates.       Zone  of  saturation  -­‐-­‐    Water  reaches  a  zone  where  all  of  the  open  spaces  in  sediment  and  rock  are   completely  filled  with  water.   • Water  within  the  pores  is  called  groundwater.   • Water  table  —the  upper  limit  of  the  zone  of  saturation   Unsaturated  zone  (Vadose  Zone)   • Area  above  the  water  table   • Open  space  is  partly  occupied  by  water     • Water  cannot  be  pumped  by  wells.   • Includes  the  capillary  fringe  and  the  belt  of  soil  moisture   Ø Belt  of  soil  moisture  —  This  “belt”  is  formed  by  water  that  is  held  by  molecular   attraction  on  soil  particles  in  the  near-­‐surface  zone   Ø Capillary  fringe  -­‐  Extends  upward  from  the  water  table.  Groundwater  is  held  by   surface  tension  in  tiny  passages  between  grains  of  soil  or  sediment.   Storage  and  Movement  of  Groundwater.    These  are  the  terms  used  to  describe  the  addition  or  loss  of   water  from  the  groundwater  system,  and  the  flow  of  water  within  it.   • Recharge  =  the  addition  of  water  to  the  ground;  where  water  enters  the  groundwater   • Discharge  =  a  place  where  groundwater  is  lost  to  the  surface   • Groundwater  flow  =  Flows  from  high  pressure  to  low  (usually  from  recharge  to  discharge  areas)   Movement  of  groundwater   • Movement  is  exceedingly  slow  —Typical  rate  of  movement  is  a  few  centimeters  per  day.   • Energy  for  the  movement  is  provided  by  the  force  of  gravity.   We  use  Darcy’s  law  to  describe  the  movement.    Darcy  was  a  French  engineer  who  lived  in  the  mid  16th   century.    Here  is  a  brief  description  of  what  his  law  means  and  the  terms  used  to  describe  groundwater   movement.   Darcy’s  law    says  that  if  permeability  remains  uniform,  the  velocity  of  groundwater  will  increase  as   the  slope  of  the  water  table  increases.    In  other  words,  if  you  think  of  the  water  as  moving  downhill,  it   moves  faster  if  the  hill  (water  table  slope)    is  steeper.   Hydraulic  gradient  is  the  water  table  slope    and  it  is  determined  by  dividing  the  vertical  difference   between  the  recharge  and  discharge  points  by  the  length  of  flow  between  these  points.    This  is  similar   to  how  you  would  determine  the  slope  of  a  hill   Hydraulic  head  —the  vertical  difference  between  the  recharge  and  discharge  points   Darcy  also  found  that  flow  velocity  varied  with  permeability  of  sediment/aquifer  –  faster  with  higher   permeability       Darcy  put  it  all  together  with  this  formula….   You  don’t  need  to  memorize  the  formula.       Discharge  (Q)  –  volume  of  water  flowing  through  an  aquifer   KA(h1 − h2 ) Q= d Hydraulic  conductivity(K)  =  a  factor  based  on  permeability  and  fluid  viscosity   h1-­‐  h2/d  =  hydraulic  gradient   Cross  sectional  area  of  aquifer  =  A     The  “bottom  line”  for  all  of  this……   With  increased  slope  (hydraulic  gradient)  and  permeability…..  have  increased  discharge   (volume  of  water  flowing  through  aquifer)     Groundwater  Elevations  (water  table)  and  Contours.    The  top  of  the  water  table  can  be  contoured,  like   a  topographic  contour  map  of  a  hill.    Groundwater  flow  is  perpendicular  to  contours.     Groundwater  Flow  Systems  -­‐local  and  regional.    Now  we  can  look  at  how  this  all  works  to  create  flow   systems  and  review  some  of  the  features  you  may  already  be  familiar  with,  like  springs,  wells  and  caves.   Springs   • Occur  where  the  water  table  intersects  Earth’s  surface   • Natural  outflow  of  groundwater   How  they  form:      These  may  form  as  an  aquitard  creates  a  localized  zone  of  saturation,  which  is   called  a  perched  water  table.    They  can  also  be  related  to  fractures  or  cave  systems  that   intersect  the  surface.       There  are  two  primary  types  of  aquifers  (unconfined  and  confined)   Aquifer  Types  -­‐  Unconfined  (Open)  Aquifer     • Recharge  occurs  over  the  entire  area-­‐  open  to  receiving  infiltration     • Change  of  Water  Table  With  Climate  (Water  table  rises  &  falls  seasonally)   There  are  3  basic  types  of  interactions  between  the  water  table  and  streams   1. Gaining  streams  —  gain  water  from  the  inflow  of  groundwater  through   the  streambed   2. Losing  streams  —  lose  water  to  the  groundwater  system  by  outflow   through  the  streambed   3. Combination  gaining  &  losing  —    Stream  gains  in  some  sections  and   loses  in  other  areas.   Important  notes:       Gaining  Streams  –  common  in  wet  /  humid  climates   Losing  Streams  –  common  in  arid  (dry)  climates   Aquifer  Types  -­‐  Confined   • Confined  aquifers  are  bounded  above  and  below  by  aquicludes  or  aquitards.     •  Water  is  under  pressure  from  overlying  rock  and  hydraulic  pressure  from  recharge.   In  confined  aquifers  we  get  artesian  wells  and  springs.   Artesian  well  —a  situation  in  which  groundwater  under  pressure  rises  above  the  level  of  the   aquifer   Types  of  artesian  wells   • Nonflowing  —  Pressure  surface  is  below  ground  level.   • Flowing  —  Pressure  surface  is  above  the  ground.   • Not  all  artesian  systems  are  wells;  artesian  springs  also  exist.   Artificial  Artesian  Pressure  for  municipal  supplies.    When  you  see  a  water  tower  or  tank  you  notice  that   it  is  high  up  in  the  air  or  in  elevation  above  the  surrounding  ground.    This  creates  an  artificial  artesian   situation,  where  the  discharge  points  in  the  water  supply  (aka  your  faucet)  are  all  below  the  water  level   in  the  tank,  creating    pressure  so  the  water  will  flow.   Here  is  some  more  information  about  groundwater  wells….   To  ensure  a  continuous  supply  of  water,  a  well  must  penetrate  below  the  water  table.   Pumping  of  wells  can  cause:   • Drawdown  (lowering)  of  the  water  table   • Cone  of  depression  in  the  water  table   Groundwater  Contamination  (Pollution).       We  can’t  review  groundwater  without  mentioning  groundwater  pollution.    This  is  a  great  concern,   particularly  in  Arizona  where  we  depend  so  heavily  on  groundwater  for  our  drinking  water  supply.   Water  can  infiltrate,  and  so  can  contaminants.    This  can  happen  from  a  variety  of  sources,  such  as   landfills,  gas  stations,  industrial  wastewater  and  domestic  wastewater  discharges,  etc.      Contaminants   percolate  downward  and  dissolve  in  groundwater.    If  enough  of  a  pollutant  percolates  downward,  some   of  it  may  not  dissolve  right  away.    Instead,  the  pollutant  may  “float”  (e.g.  gasoline)  or  “sink”  (e.g.   chemical  solvents)  depending  on  whether  they  are  lighter  (less  dense)  or  heavier  (more  dense)  than   water.      Once  contaminants  dissolve,  groundwater  and    pollutants  flow  “downgradient”in  what  is  called   a  “plume”.    Pumping  can  also  create  a  lowering  of  the  water  table,  which  can  draw  pollutants  into  a   well.    The  aquifer  type  also  plays  a  role  in  the  movement  of  pollutants.    For  example,  there  can  be  rapid   movement  in  fractured  limestones,  leaving  less  time  for  the  groundwater  to  be  “purified”.    Movement  is   slower  in  sandstones,  leaving  more  time  for  purification.    In  addition,  some  pollutants  degrade/break   down  more  quickly  than  others,  and  many  break  down  naturally.    However,  it  can  take  decades  or  much   longer  for  some  to  degrade.    Once  contaminants  are  located,  decisions  can  be  made  about  protecting   drinking  water  supply  wells,  and  whether  or  not  the  additional  cleanup  (remediation)  is  necessary.    If   natural  processes  are  not  enough,  man-­‐made  technologies  may  need  to  be  used  to  reduce  or  remove   the  contaminants.   Saltwater  contamination  is  another  problem  that  can  be  associated  with  groundwater  withdrawl.   • Excessive  groundwater  withdrawal  causes  saltwater  to  be  drawn  into  wells  (saltwater   intrusion),  thus  contaminating  the  freshwater  supply.   • Primarily  a  problem  in  coastal  areas   Additional  Problems  Associated  with  Groundwater  Withdrawal   • Treating  groundwater  as  a  nonrenewable  resource.    We  need  to  understand  that  groundwater   is  not  in  unlimited  supply.    In  many  places,  the  water  available  to  recharge  the  aquifer  falls   significantly  short  of  the  amount  being  withdrawn.   • Subsidence  -­‐  The  ground  sinks  when  water  is  pumped  from  wells  faster  than  natural  recharge   processes  can  replace  it  (called  “overdraft”).    This  is  a  problem  in  many  areas,  including  Arizona.     Earth  cracks  (fissures)  can  be  formed,  particularly  in  arid  climates.     Geologic  Work  of  Groundwater   Groundwater  dissolves  rock  –  REVIEW.    Remember  that…..   • Groundwater  is  often  mildly  acidic.   • Contains  weak  carbonic  acid    -­‐  Forms  when  rainwater  dissolves  carbon  dioxide  from   the  air  and  from  decaying  plants   • Carbonic  acid  reacts  with  calcite  in  limestone  to  form  calcium  bicarbonate,  a  soluble   material.   Caverns   • Most  caverns  are  essentially  “holes/empty  space”  created  by  acidic  groundwater   dissolving  soluble  rock  (calcite/limestone)  at  or  just  below  the  surface  in  the  zone  of   saturation.     • They  form  in  the  zone  of  saturation.   Features  found  within  caverns   • Composed  of  dripstone  (travertine)   • Calcite  deposited  as  dripping  water  evaporates.   • Collectively,  they  are  called  speleothems.     • Includes  stalactites  (hanging  from  the  ceiling)  and  stalagmites  (form  on  the   floor  of  a  cavern)   Karst  topography    -­‐  Landscapes  that  have  been  shaped  mainly  by  the  dissolving  power  of   groundwater   Some  common  features  include:   • Irregular  terrain   • Sinkholes  or  sinks  (formed  by  groundwater  slowly  dissolving  the   bedrock  often  accompanied  by  collapse).    Sinkholes  can  develop  when   cave  roofs  collapse.    Sometimes  this  has  occurred  in  residential  areas   (e.g.  in  Florida)   • Striking  lack  of  surface  drainage  (streams)   Hot  Springs  and  Geysers   Hot  springs   • Water  is  6  oC  to  9  oC  warmer  than  the  mean  annual  air  temperature  of  the  locality.   • The  water  for  most  hot  springs  is  heated  by  the  cooling  of  igneous  rock.   Geysers   • Intermittent  hot  springs   • Water  erupts  with  great  force.   • Occur  where  extensive  underground  chambers  exist  within  hot  igneous  rock   • Groundwater  heats,  expands,  changes  to  steam,  and  erupts.   Geothermal  energy  –  Just  as  geysers  are  created  from  volcanism,  steam  can  be  used  in  areas  that  are   naturally  hot  to  drive  turbine  generators  to  produce  electricity.    For  example,  in  Iceland,  which  is  located   above  the  mid-­‐Atlantic  ridge,  geothermal  is  a  major  source  of  energy.