Mahon Creek Water Monitoring Station

Overview

Mahon Creek originates south of Highway 57 and flows through farm fields and a housing development onto the UW-Green Bay campus where the stream flows through Mahon Woods and into the bay of Green Bay. The Mahon Creek watershed lies within the Lower East Shore watershed. The map below shows the boundary of the Mahon Creek watershed and the position of the water monitoring station. Due to rapidly shifting variables, a continuous automatic monitoring system was needed to accurately characterize hydrology and water quality in the stream. A permanent station was installed in December of 2010 with the help of staff, students, and several generous donors. Data collected at this station provide an important educational resource for students and is used to help manage water quality on the UW--Green Bay campus, Mahon Creek watershed and the bay of Green Bay. The station is maintained by Cofrin Center for Biodiviersity staff Bobbie Webster and Andrew Laplant, with help from volunteers.

East Shore Watersheds

Stream height (stage) is monitored continuously in Mahon Creek and in an urban stormwater culvert located 75 feet away from the stream. Stream discharge (volume/time) is periodically measured manually with an OTT acoustic digital current (ADC) meter, and stage is recorded at the same time as the flow measurement. The resulting stage-discharge relationship is combined with continuous stage data to calculate continuous stream discharge. Other sensors record water temperature, pH, dissolved oxygen and conductivity continuously during ice-free periods. In July 2012 a Campbell Scientific turbidity probe was installed in the urban culvert which recorded turbidity at 5 minute intervals during storm events, and less frequently at other times.  This allowed UW-Green Bay researchers to establish a relationship between turbidity and SSC, particulate P and total P that was used to estimate concentrations and constituent loads on a continuous basis.

Mahon Reseach Station House

During run-off events such as during and after storms or during major snow melt, samples are automatically collected based on the rise and fall in stream height. Samples are also collected manually during low flow conditions. Samples are analyzed in the UW-Green Bay chemistry laboratory for Total Phosphorus (TP), Dissolved Phosphorus (DP) and SSC (suspended solids concentration).  A sufficient number of samples have been collected for researchers to calculate daily loads when the data are coupled with our continuous flow record. 

The permanent station was constructed in winter 2010/2011. The shingled building that houses the sampling and electrical equipment was designed to blend in with the surrounding forest.

Mahon Creek during flood event

Equipment and measurements

  • Continuous stream height (stage) in Mahon Creek is measured with an OTT bubble sensor pressure transducer (ongoing). Stream stage downstream from the urban culvert is recorded with a Druck pressure transducer (ongoing). A Campbell Scientific CR1000 programmable data logger is used to record all data, with remote access provided via a cell-phone modem.
  • Stream discharge (volume/time) is periodically measured manually with an OTT acoustic digital current (ADC) meter. Stream discharge can also be measured periodically with a StreamPro acoustic velocitiy meter that is mounted on a small boat and towed across the stream during larger storm events that are difficult or impossible to measure instream.
  • Water temperature, pH, dissolved oxygen and conductivity were recorded continuously (2011 to 2016) with Hydrolab Multiparameter sondes located within each stream and data are transmitted through a direct cable connection to the Mahon Station.
  • Continuous turbidity was collected urban culvert adjacent to the station through 2015 with a Campbell Scientific OBS-500 turbidity probe. Turbidity from the probe can be used as a surrogate to estimate continuous SSC and TP concentrations.
  • Runoff event samples (TP, DP and SSC) are collected from Mahon Creek throughout an event with a refrigerated Teledyne ISCO automated sampler. Grab samples are periodically collected during non-event periods. TP and DP are analyzed with the Lachat block digestion and Flow Injection Analysis system either at UWGB or a commercial lab such as NewWater.
  • Continuous stream discharge has been calculated and daily loads of TP and SSC have been calculated with USGS GCLAS software.
  • Reference to any specific commercial monitoring products or manufacturer does not imply its endorsement, recommendation or otherwise by UW-Green Bay.

Results

Concentrations of TSS, TP and DP (mg/L) in water samples collected from Mahon Creek are summarized in Table 1 for event, low flow, and combined sample categories. over 800 samples have been analyzed thus far for TP and TSS, and 156 samples for DP. The maximum concentration of TP was 5.8 mg/L, with 65 samples of 0.9 mg/L or greater (7.6% of all samples). The mean concentration of TSS was 147 mg/L with a maximum of 3,520 mg/L.  Median concentrations were 0.120 mg/L for TP and 0.117 mg/L for DP during May to October 2019, compared to the TP standard of 0.075 mg/L (6 samples with lower limit of detection for phosphorus analysis).

Table 1. Summary of water sample concentrations at UW-Green Bay Mahon Creek Research Station (mg/L).

a) Mahon Creek UW-Green Bay Research Station near outlet to Green Bay

  ALL samples   EVENT samples   LOW flow samples
  TP DP TSS   TP DP TSS   TP DP TSS
# samples 859 162 819   740 74 701   117 88 117
Mean 0.324 0.091 147   0.362 0.136 170   0.087 0.052 11
Median 0.156 0.048 41   0.195 0.081 58   0.062 0.038 6
Max 5.78 1.77 3520   5.78 1.77 3520   1.14 0.29 154
#>0.5 mg/L 136 2     135 2     below: May-Oct only and with lower detection limits
#>0.9 mg/L 65 2     64 2     14 14 # samples
#>2.0 mg/L 19       19       0.125 0.095 mean
#>3.0 mg/L 7       7       0.11 0.072 median
  ALL samples   EVENT samples   LOW flow samples
  TP DP TSS   TP DP TSS   TP DP TSS
# samples 859 162 819   740 74 701   117 88 117
Mean 0.324 0.091 147   0.362 0.136 170   0.087 0.052 11
Median 0.156 0.048 41   0.195 0.081 58   0.062 0.038 6
Max 5.78 1.77 3520   5.78 1.77 3520   1.14 0.29 154
#>0.5 mg/L 136 2     135 2     below: May-Oct only and with lower detection limits
#>0.9 mg/L 65 2     64 2     14 14 # samples
#>2.0 mg/L 19       19       0.125 0.095 mean
#>3.0 mg/L 7       7       0.11 0.072 median

b) Mahon Research Station: Urban Culvert

  ALL samples
  TP DP TSS
# samples 72 53 70
Mean 0.084 0.052 17
Median 0.040 0.035 2
Max 1.03 0.60 932

NOTE: Only 3 culvert events sampled

Water sample concentrations and annual loads are summarized in the below tables and figure.

Summary of water sample concentrations (mg/L) at UW-Green Bay Mahon Creek Research Station near outlet to Green Bay

  ALL samples   EVENT samples   LOW flow samples
  TP DP TSS   TP DP TSS   TP DP TSS
# of samples 859 162 819   740 74 701   117 88 117
Mean 0.324 0.091 147   0.362 0.136 170   0.087 0.052 11
Median 0.156 0.048 41   0.195 0.081 58   0.062 0.038 6
Max 5.78 1.77 3520   5.78 1.77 3520   1.14 0.29 154
#>0.5 mg/L 136 2     135 2     below: May-Oct only and with lower detection limits
#>0.9 mg/L 65 2     64 2     14 14 # of
samples
#>2.0 mg/L 19       19       0.125 0.095 mean
#>3.0 mg/L 7       7       0.11 0.072 median
The median TP concentration for Mahon Creek during the May to October 2019 period was 0.120 mg/L (n = 6), which exceeds the Wisconsin phosphorus criteria of 0.075 mg/L (Figure 1). The median DP concentration was 0.117 mg/L, indicating that most of the phosphorus is readily available for uptake by algae during low flow conditions. This period and statistic are consistent with the methodology used to establish the phosphorus criteria (Robertson et al, 2006, 2008); however, this methodology involved random sampling near mid-month, rather than only during non-event periods as was used in this project. If event data had been included in this project, the median concentrations may have increased. Only data collected from September 2018 onward were summarized for the May to October period because we recently started utilizing a phosphorus analysis method with a lower detection limit at the certified lab where the samples were submitted for analysis.
 

Figure 1. May to October total phosphorus concentrations under low flow conditions in Mahon Creek and nearby Wequiock Creek, relative to the Wisconsin water quality standard.

Annual Loads TP TSS Flow
  (kg)  (Mg) (mm)
2011 704 228 355
2012 179 88 188
2013 297 76 298
2014 363 139 319
2015 401 104 231
2016 401 126 375

Comparisons to streams in other Lower Fox watersheds

Urban Culvert

Samples were also collected just below the outlet of a nearby 6-ft (1.8 m) urban culvert, which is adjacent to the UW-Green Bay research station. This culvert drains 0.873 mi2 (2.26 km2) of mostly urban area within the City of Green Bay and joins the main stem of Mahon Creek just downstream of the primary stream monitoring location. Sample results from the urban culvert are summarized in Table 1b and indicate that phosphorus concentrations from the culvert (n = 72) are similar to those collected from the adjacent main stem of Mahon Creek during low flow conditions.

Mahon Research Station - Urban Culvert: water sample concentrations (mg/L).
  TP DP TSS
# samples 72 53 70
mean 0.084 0.052 17
median 0.040 0.035 2
max 1.03 0.60 932
Mahon nearby Urban Culvert

Additional Data Sources

We would like to thank the following generous donors for contributing funds, equipment, materials, and labor toward the construction and operation of the Mahon Creek Station:

  • Cofrin Center for Biodiversity
  • 1923 Fund (primary donor)
  • Van's Lumber & Custom Builders (materials and labor)
  • Wisconsin Public Service Corporation (sondes)
  • Prefinished Staining Products, Inc. (siding)
  • UW – Green Bay Facilities Department
  • USGS - Wisconsin Water Science Center (equipment and expertise)