Agriculture Reference
In-Depth Information
study of flow patterns in an outwash plain located ~35 km southwest of KBS
(Kehew et al. 1996). For the Augusta Creek watershed near KBS, spatial patterns
of groundwater flow have been inferred from maps of the water table in unconfined
aquifers, as derived from water-supply well records (Bartholic et al. 2007). In the
land surrounding Gull Lake (Fig. 11.1B), the elevation of the water table varies
only about 40 m; the water table around the MCSE lies about 30 m higher than the
lowest land-surface elevation in the area (along the Kalamazoo River).
Hydrochemical information on local groundwaters is available mainly from
analyses of domestic wells and is summarized in Table 11.1 (Allen et al. 1972,
Rheaume 1990, Kehew and Brewer 1992). Most domestic wells in the area pump
groundwater from relatively shallow depths of <25 m, so the analysis of well water
samples provides an indication of the hydrochemistry of groundwaters that would
be discharged to lakes and streams. The major ion composition of a well close to the
KBS LTER main site is depicted in Fig. 11.9. As discussed above, the dissolution
of carbonate minerals is largely responsible for the high ionic strength of ground-
water around KBS (Kehew et al. 1996), as it is throughout the lower peninsula of
Michigan (Wahrer et al. 1996, Jin et al. 2008b). Compared to the water sampled
from the soil profile at a depth of 1.8 m (Fig. 11.6), which is just into the carbonate
mineral zone, groundwater has higher concentrations of Ca
2+
, Mg
2+
and HCO
3
−
, and
higher ratios of Mg
2+
to Ca
2+
(Jin et al. 2008b, 2009). Groundwater also often has
higher SO
4
2−
concentrations than the soil waters measured at KBS; SO
4
2−
in ground-
water can originate from gypsum (CaSO
4
) dissolution or from oxidation of sulfide
minerals (e.g., pyrite FeS
2
) in the glacial deposits (Böhlke 2002).
Local groundwaters can generally be classified as the calcium-magnesium-
bicarbonate type due to the dissolution of carbonate minerals. Ferrous iron concen-
trations may be elevated in groundwaters with little or no dissolved oxygen. These
concentrations likely increase during the passage of recharge waters through zones
of low redox potential such as lake and wetland sediments (Kehew et al. 1996) or
when there is very high organic loading at the soil surface.
In agricultural landscapes, NO
3
−
concentrations in domestic wells commonly
exceed the drinking-water standard of 10 mg N L
−1
(Rupert 2008, Puckett et al.
2011), although the KBS well water shown in Fig. 11.9 had a particularly low
NO
3
−
concentration (13 µg N L
−1
= 1 µeq L
−1
) compared to other wells in the area.
Chowdhury et al. (2003) analyzed data for 8733 wells in Kalamazoo County
(where KBS is located) and found that NO
3
−
concentrations exceeded 5 and 10 mg
N L
−1
in 28% and 3% of the wells, respectively; concentrations >5 mg N L
−1
were
considered indicative of surface contamination sources. Rheaume (1990) presented
evidence that NO
3
−
concentrations in the area have increased substantially over
the past few decades, documented the spatial patterns in NO
3
−
contamination as of
the late 1980s, and formulated an N budget that implicates agricultural fertilizers
as the primary cause of the increased contamination of local aquifers. As men-
tioned above, atmospheric deposition is a substantial source of N loading as well.
Groundwater in the deeper layers of the glacial deposits still tends to be lower in
NO
3
−
(Rheaume 1990, Kehew et al. 1996), which likely reflects its longer turnover
time and the relatively recent history of fertilizer use in the region as well as reac-
tions that consume NO
3
−
in the groundwater system (Böhlke 2002).
