BHI Water Quality AssessmentDr. Beth Darrow, Senior Scientist
UPDATE: As of 10/11/2018, the swimming advisory for BHI has been lifted.
Upon returning to the island this week, we conducted an immediate visual and water quality assessment on the creek sites that we normally monitor weekly, and additional beach sites around the island. The tide was flooding (coming in) at the time of sampling on October 2, which was from 10 am - 2 pm. Ocean water (east beach) was mixing with river water (salinity was lower than the usual 35 ppt of ocean water), but we did not see the effects of river water until Access 11, where we saw a sudden drop-off in salinity and dissolved oxygen due to the influence of river water.
Fresh water was draining from the marshes into the upper reaches of the creeks, which had very low dissolved oxygen. River water at West Beach had intermediate salinity and dissolved oxygen between East Beach and the Creek headwaters. It is likely that during low tides at night, the creek is going anoxic (no oxygen). We do not have data back yet from nutrient, carbon, or bacteria samples that we took yesterday, but will have those data in the next few weeks. We will also be conducting a 24-hour sampling of creek water to assess overnight dissolved oxygen levels.
Dissolved oxygen is low due to organic matter and nutrients coming downstream, in both the river water and creek. Some of this organic matter is natural - leaves, marsh grass, pine needles… some of it is from humans - sewage, fertilizer, road run-off. It all gets respired by bacteria, using up more oxygen than is being produced by plants through photosynthesis. Our island is certainly seeing the effects of being in the mouth of the Cape Fear River, since our creek water is not getting much extra oxygen from the river when the tide comes in.
Our creek animals need oxygen to survive. Usually, fish can move away from low dissolved oxygen, but sessile animals cannot. In low oxygen environments, often a shift is seen from more-sensitive to less sensitive animals (fewer mollusks, more worms, for example).
As of 10/06/2018 there is a still a swimming advisory for Bald Head Island. Swimming is not recommended in the ocean near areas where floodwaters are draining off island, specifically along South Beach. Bacteria levels can be very high near these locations. East Beach waters tested clean. The Conservancy will take samples for the state testing of recreational waters next week. Status of BHI beaches will be posted Wednesday.
Surface Water Quality Data
Tuesday, October 2, 2018:
Bald Head Creek
Salinity range = 6.4 - 26.7 psu
Dissolved oxygen = 29.4 - 57 % (2.28 - 4.08 mg/L)
Salinity = 4.96 psu
Dissolved oxygen = 49 - 55% (3.91 - 4.36 mg/L)
South Beach (Accesses 11, 17, 34)
Salinity range = 26.47 - 26.82 psu
Dissolved oxygen = 77 - 94% (5.42 - 6.09 mg/L)
East Beach (Access 42)
Salinity = 27.16 psu
Dissolved oxygen = 80.1% (5.41 mg/L)
West Beach (Accesses 1, 8)
Salinity range = 13.98 - 16.22 psu
Dissolved oxygen = 34.8 - 59.1 % (2.53 - 4.21 mg/L)