Heather L. Welch US Geological Survey, Lower Mississippi-Gulf Water Science Center

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1 Contribution of total dissolved phosphorus in irrigation runoff from the Mississippi River valley alluvial aquifer to phosphorus concentrations in a Delta stream Heather L. Welch US Geological Survey, Lower Mississippi-Gulf Water Science Center

2 Phosphorus in groundwater Concentrations are typically low in groundwater National background is 0.03 mg/l (Dubrovsky and others, 2010) Groundwater can influence surface-water quality Streams that consist primarily of groundwater inflow Irrigation return flow USEPA desired goal for phosphorus in streams is 0.1 mg/l for the prevention of nuisance plant growth

3 Alexander et al., 2008 Jacobson, 2010 Average annual county level P application,

4 OCCURRENCE OF PHOSPHORUS IN THE ALLUVIAL AQUIFER

5 Mississippi River Valley alluvial aquifer Underlies portions of Arkansas, Mississippi, Tennessee, Missouri, Louisiana, Kentucky, and Illinois Mississippi pumps about 1.3 billion gallons per day Reducing conditions in the aquifer Low dissolved oxygen (<0.5 mg/l) High concentrations of iron, sulfate, and manganese

6 Two subunits of the MRVA There are two subunits of the MRVA Holocene alluvium thicker overlying clay layer Pleistocene valley trains coarser sediments Higher median P concentrations were found in the Holocene alluvium in a 1998 NAWQA study Most prevalent subunit in Mississippi Aquifer Minimum Concentration (mg/l) Maximum concentrations (mg/l) Median concentration (mg/l) Holocene alluvium < Pleistocene valley trains <

7 2010 study MS Delta was divided into cells using an equal-area method 46 wells were sampled from May to October 2010 All irrigation wells Depths ranged from 70 to 140 feet Phosphorus was detected in 100% of the wells Concentrations ranged from 0.12 to 1.17 mg/l Median = 0.62 mg/l

8 Almost 2X the streamflow in September. This could be a contribution from irrigation return flow. Lowest streamflow, highest percent of DP could indicate groundwater source from gw discharge to streams Most of the year DP contribution to total phosphorus has a median of 20% or less At low flow, DP can compose as much as 70% (median) of TP Highest percent contributions occur from July to October

9 CURRENT WORK

10 Project Purpose Mineralogical analysis of the alluvial aquifer sediments Field-scale study to determine the effect of dissolved phosphorus from irrigation runoff on surface-water quality

11 Rice Field Study Forestdale Well Riser Stream Sharkey clay

12 Rice Field Study May through August samples were collected Irrigation well Stream that drains the field Also one of the 319 sites so there is historical data 9 samples were collected from a riser located at the outlet of the rice field to the stream Irrigation started later last year and was not used as much as in years past due to timely rain during the growing season

13 0.18 Total dissolved phosphorus Concentration, in mg/l Stream Riser Well

14 Dissolved phosphorus, in mg/l Chloride concentration, in mg/l Stream Riser Well

15 Percent DP of the TP Discharge, in cubic feet per second

16 Data interpretation Still in the early phases of interpretation Initial results for this site seem to indicate that the dissolved phosphorus content is increasing as the water moves across the field This is likely not the case Delta-wide corn, soybean, and cotton fields don t use flood irrigation. Historical soil data I would like to see if the phosphorus content in the Delta soils showed an increase after irrigation began. I need pre and post-1980s soil data. Are the phosphorus concentrations in the soil a result of historic applications of P fertilizer that have moved through the unsaturated zone into the alluvial aquifer? Have the Delta soils always been high in phosphorus content? If so, does irrigating from the alluvial aquifer just provide a constant source/exchange with the phosphorus that is already present. What happens in streams where groundwater is still discharging to Delta streams?

17 Year 2 Mineralogical analysis 11 cores are going to be analyzed for mineralogical analysis using X-ray diffraction (XRD) 3 surficial soil samples are going to be analyzed for mineral content Isotopic analysis of water from the alluvial aquifer and surface water in the Delta at 3 different locations Oxygen isotope of phosphate (δ 18 O p ) Hydrogen and oxygen isotopes