How Did Se Get Into The Marshes?


California's San Joaquin Valley is a broad basin that collects sediment washed by streams and rivers from the Coastal Ranges to the west and the Sierra Nevada mountain range to the east.


Many trace elements come from the ocean.

Many of the rocks in the hills that border the San Joaquin Valley, particularly on the west, originated in the ocean. What trace elements would you expect these rocks to contain

For tens of thousands of years, trace elements and salts have become concentrated by evaporation in the hot climate of the valley.

Irrigation causes trace elements to move.

Extensive use of irrigation has made the San Joaquin Valley one of the world's most productive farming areas. Unfortunately, it has also washed concentrations of salts, minerals, and trace elements into wetlands. Here's what happens:


  1. Farmers use irrigation water to flush excess salts from the soil. (This practice removes salt and trace elements from the root zone.)
  2. Salts and trace elements filter into the groundwater.
  3. Drainage systems carry away the contaminated runoff water.
  4. Subsurface drains carry away salts, along with Se and other trace elements.
  5. Runoff water is collected via drains, surface runoff and, groundwater into low-lying marshes and ponds, where it is allowed to evaporate.
Click here to try an activity that explores this idea further:


CLUE #2: Naturally occurring Se became concentrated in the marsh because the agricultural runoff contained Se and other trace elements that had been concentrated in the soil by thousands of years of evaporative processes. Once in the marsh, the water could only leave by evaporation or percolation into the ground water, leaving most of the Se and other trace elements and salts behind.

Wetlands Introduction

Clue #1

Clue #3

Clue #4

Putting the Pieces Together


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