Middle St. Johns - Florida Department of Environmental Protection

Water Quality Assessment Report: Middle St. Johns 467
17. Lake Ivanhoe (WBID 2997F) is shown as being impaired for dissolved oxygen. The
data presented in the IWR_datbase_Run_11a shows that 25 of the 153 dissolved
oxygen samples collected in Lake Ivanhoe are impaired, which is above the minimum
number of violations (21) required to place dissolved oxygen on the verified list.
However, information provided by the City of Orlando indicates that these violations
are due to the presence of bottom dissolved oxygen values in STORET and the
IWR_database. It is the practice of the City of Orlando to collect dissolved oxygen
data from 0.5 meters below the water surface to the bottom of the lake at one meter
increments and enter both surface and bottom dissolved oxygen values into the
STORET database. Bottom depth dissolved oxygen data are recorded in STORET
for the City’s use in documenting the thermal stratification for all City lakes.
However, during a review of this data, it became apparent that corresponding sample
depth was not recorded with the dissolved oxygen values and consequently used in
the determination of dissolved oxygen for this water body Instead of just the surface
water measurement. The water quality data collected by the City (21FLORL)
provided in Appendix D indicates that 83 of the 108 samples collected between 1996
and 2002 represent surficial oxygen concentrations (0.5 feet below the surface). The
minimum surficial dissolved oxygen is 5.62 mg/L. As the 25 bottom samples taken at
the sediment water interface in the deepest portion of the lake do not represent
oxygen levels in the overlying water column, these values have been identified in
Appendix D for removal from the IWR database. This will help better representing
actual lake conditions. It should be noted that the presence of oxygen at the sediment
interface is another indication of this water body healthy conditions. Without the
bottom samples, there are no water quality standard violations during the past 7.5
years to impair this WBID for dissolved oxygen. Recommendation: Remove the
Dissolved Oxygen Impairment for Lake Ivanhoe from the Verified List [Orange
County TMDL Analysis 5 PBS&J 100504.01 July 2003].
RESPONSE: As of the September 2, 2003 update of IWR database, there were 32
instances of dissolved oxygen not meeting the 5 mg/L criterion out of a total of 153
observations in the verified period for this lake. None of the 32 observations below 5
mg/L had associated depth values provided, as noted above. Most of the values
above 5 mg/L however had depths associated with them. It is understood that the
data without depths were not intended for general characterization of the lake (City of
Orlando use only) and these values can be removed from the IWR database and
impairment assessment of this segment. Orange County informed the Department of
which data need to be removed from the IWR database, and these data were removed
in the November 5, 2003 update of the IWR database, therefore this segment is not
listed for dissolved oxygen.
18. Lake Lotta (WBID 3002G) is shown as being impaired for nutrients (TSI). However,
based upon a review of the data, six annual average TSI data values measure below
60 and the annual TSI values suggest an improving trend. This does not support the
impairment conclusion reached for this water body. Recommendation: Remove the