Salinity is the measurement of salt dissolved in water. Salinity is usually measured in parts per thousand (reported as “ppt”). The average ocean salinity is 35 ppt: this means that in every kilogram of seawater (1000 grams), 35 grams are dissolved salt. Waters are considered fresh if they have less than 2 ppt. Lake Pontchartrain water is brackish, meaning it is neither completely fresh nor as salty as seawater.
Water sensor measurements at the Lighthouse were also continuously made both before and after the landfall of Hurricane Ida on August 29, 2021:
- Measurements of specific conductance, which also represents how much dissolved salt is in water by measurements of how well electricity is conducted through it, also provided interesting information about changes in water quality in the lake after the arrival of Hurricane Ida. Specific conductance is measured in microsiemens per centimeter, or uS/cm – the higher this number is, the saltier the water. At 5pm on August 30, specific conductance levels began to rise steadily from 1000uS/cm until a high of 4160uS/cm was reached during the early morning of September 4. These observations are consistent with what was expected given the storm’s movement through the region - the east to west movement of saltier Gulf of Mexico waters through Lake Borgne and into Lake Pontchartrain that accompanied the storm’s eastern surge as its center moved to the south and west of New Orleans. The intrusion of these saltier waters appears to have been slightly delayed in reaching the Lighthouse but lingered well past the storm’s passage.