Prior Lake - Spring Lake Watershed District 2009 Water Quality ...

. TurbidityTurbidity is a measurement of water clarity or cloudiness. The higher the turbidity value, thecloudier the water. Turbidity is affected by the concentration and type of suspended materialin the water, such as soil particles and/or algae.The state water quality standard for turbidity is 25 NTU’s (Nephelometric Turbidity Units).However, the sondes used for monitoring turbidity in the PLSLWD measure in FNU’s (FormazinNephelometric Units). Very few turbidity‐monitoring devices actually measure in NTU’s. Forexample, turbidity can also be measured in NTRU’s, JTU’s, FTU’s, and even more depending onthe type of equipment used. The values produced from different equipment is often similar,but not interchangeable due to the various methods available for measuring turbidity (angle ofdiffraction, light source used, etc). Further studies would need to be conducted to find a truerelationship between FNU’s and NTU’s, but each site can have a different relationshipdepending upon the suspended particle properties (i.e. color, shape, and reflectivity ofparticles) causing the turbidity. This issue continues to be an on‐going discussion and will mostlikely not be resolved in the near future; however, the Minnesota Pollution Control Agency(MPCA) is working on resolving this issue. For this study, it was assumed that 25 NTU’s are veryclose to 25 FNU’s.Figure 6 displays the maximum turbidity values recorded during the synoptic monitoring in2009. Sites #19, #28, and #40 had the highest turbidity values. Site #11 (Buck Lake Inlet) alsohad extremely high turbidity values that were discovered in the deployment study (see figure5). PLSLWD staff will look into possible solutions for reducing turbidity in all of these locations.Page 9 of 15