2.  Offshore    intakes.   Offshore   intakes   are sometimes  required  because  of  lack  of  time, personnel, or equipment or because of coral con- ditions that prohibit well construction. Intakes of either the rigid pipe or float type may be used but they should be located in deep water beyond the surf.  They  must  be  positioned  vertically  and  be off the bottom but still beneath the water surface at low tide. In this way foreign materials in the water  which  might  cause  excessive  wear  on  equip- ment  will  be  largely  excluded.  The  rigid  pipe intake  can  be  placed  on  timber  supports  and anchored securely in position by piling or riprap. Floats securely anchored can support the intake screen  in  much  the  same  manner  as  in  surface waters. A rubber suction hose can be used to con- nect the rigid pipe on the sea bottom to the pipe supported  beneath  the  float. WATER  CONTAMINATION Water  takes  on  various  characteristics  and properties as it passes over and through the earth. These characteristics and properties vary and are dependent  on  the  materials  encountered.  These materials may be natural or man-made and are classified  according  to  their  means  of  detection. .  Physical—detected  by  one  or  more  of  the human senses .  Chemical—detected  by  chemical  analysis Biological—detected   by   testing   for chloroform   organisms .   Radiological—detected   by   radiac   equip- ment  and  special  laboratory  field  tests PHYSICAL  IMPURITIES Physical   impurities   in   water   are   either suspended  or  dissolved.  The  suspended  impurities are   usually   more   dangerous   to   health.   They include  mineral  matter,  disease  organisms,  silt, bacteria,  and  algae.  They  must  be  destroyed  or removed  from  water  that  is  to  be  consumed  by humans. The  most  important  physical  characteristics are turbidity, color, odor, taste, and temperature. Valuable   information   can   be   obtained   by observing the water with any of the five human senses and using commonsense judgment on the following   characteristics: l   Color .   Turbidity .   Odor .  Taste  (use  with  caution) .  Temperature .  Condition  of  vegetation  around  source (dead  or  mottled  vegetation  can  indicate the  presence  of  chemical  agents) .  Presence  of  dead  fish,  frogs,  and  so  forth Before  starting  any  treatment  to  remove color,  turbidity,  taste,  or  odor,  you  should  take several preventive measures. You  must  prevent  the  formation  of  algae  in raw water supply points. Algae can be controlled with  copper  sulfate,  chlorine,  or  activated carbon.  Before  deciding  which  method  or  com- bination  of  methods  maybe  most  effective,  con- sider  the  following  factors: .  Volume  of  water  to  be  treated l  Time  of  year .  The  effects  of  treatment  on  fish  life . Type of secondary water treatment in use .  Equipment  available l  Cost  of  treatment You must also prevent the raw water source from  becoming  polluted  by  drainage  from  in- dustrial waste and surface drainage from farms, mines,  and  watersheds. The above conditions usually cause water to take  on  color  due  to  the  presence  of  colored substances in solution, such as vegetable matter dissolved   from   roots   and   leaves.   Dissolved humus,  iron,  and  salts  could  also  be  included. True  color is due to substances in true solution, apparent   color  includes  true  color  and  substances suspension   rather   than   dissolved.   Color   may also  be  caused  by  industrial  wastes  and  turbidity. Color   as   such   is   harmless,   but   objectionable 9-14