201Journal of Paleolimnology 23: 201–205, 2000.© 2000 Kluwer Academic Publishers. Printed in the Netherlands.

An inexpensive, optical (infrared) detector to locate the sediment/waterinterface in lakes with unconsolidated sediments*

Peter Myers1 and Claire L. Schelske1,2

1Department of Fisheries and Aquatic Sciences 7922 NW 71st Street Gainesville, FL 32653 USA (E-mail:gvlpete@worldnet.att.net & schelsk@nersp.nerdc.ufl.edu2Author for correspondence

Received 30 June 1998; accepted 18 February 1999

Key words: coring, method, Lake Apopka, Newnans Lake, depth sounding, shallow lakes

Abstract

Unconsolidated, flocculent sediments that are frequently resuspended by wind action are found in many shallow-waterlakes. Collecting sediment/water interface cores in such lakes for paleolimnological study may be problematic becauseit is difficult to determine the depth to the water/sediment interface. Accurately determining this water depth is necessaryto guarantee that a piston corer does not penetrate the sediments prior to the drive and to maximize the core length. Asimple instrument constructed with inexpensive, readily available components is described. This infrared floc detector(IFD) is used to sense the increased optical density of unconsolidated sediments as the detector is lowered into a lake.The IFD, in effect, yields a precise as well as an accurate measure of water depth. The depth to the water/sedimentinterface can be measured with an accuracy of approximately 1 cm, provided surface waters are relatively calm.

Introduction

Soft, unconsolidated (flocculent) sediments are charac-teristic of many shallow lakes in Florida (Binford &Brenner, 1988). These deposits are highly, organic(> 60% organic matter) and thus differ from sedimentsin other regions of the world. Locating the water/sediment interface in these soft, flocculent sedimentsis problematic due to their high porosity. Collectingpiston cores, therefore, is complicated because it isdifficult to locate the water/sediment interface and thusto guarantee that the mud/water interface will beretrieved with every core. Here we describe a simple,inexpensive instrument based on infrared optics thatcan be used to locate the position of the water/sedimentinterface quickly and accurately in high porositysediments or in other sediments with a surface layer ofunconsolidated material.

Several methods are employed to ascertain the water/sediment interface prior to coring. The coring assembly

*Journal Series No. R-06405 of the Florida Agricultural Experi-ment Station

can be lowered until there is some resistance from thesediments and then be raised and moved laterally beforethe coring drive is completed. Other tools to measure thewater depth to the water/sediment interface include anacoustic sounder (electronic depth finder) and aweighted, flat surface such as a Secchi disc. Usingthese devices to measure water depth in lakes withunconsolidated sediments may not be entirely satis-factory due to problems illustrated in Figure 1. Theweighted, flat disc lowered to the bottom with a light lineor rope may sink into the sediment, particularly if theplane of the disc is not parallel to the water/ sedimentinterface or if the disc is lowered rapidly. The disc, also,may disturb the sediment when it is raised. The problemwith the acoustic sounder is that sound is reflected fromdenser sediment layers below the water/sediment inter-face, which also results in a false overestimate of waterdepth. These problems in estimating water depthsaccurately in lakes with unconsolidated sediments areeliminated with the infrared floc detector (IFD).

In this paper we provide instructions for fabricationand assembly of an IFD. Construction is simple andinexpensive. Cost of materials is approximately US $40.

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Floc detector construction and theory ofoperation

The optical IFD operates on the principle of usinginterruption of the light path between a light source anda sensor to trigger an electronic signal. Unconsolidatedsediments (floc) in Florida lakes, including LakeApopka (Schelske, 1997), have a viscosity and densitysimilar to water, but have very different opticalproperties. The interface between water and theseunconsolidated sediments, therefore, can be sensedelectronically by detecting the interruption of aninfrared light beam. The detector uses an infrared diodeas the light source and a photo-transistor as thereceiver. The infrared diode and photo-transistor areplaced 2.0 cm apart to minimize light absorption bywater, yet close enough so the beam is sharply inter-rupted by flocculent sediments. We designed the lightpath in our detector head to be adjustable for experi-mental purposes, and found a 2.0 cm gap worked wellunder all tests (see Figure 2). The detector head ismade from common brass tubing available at hobbyor hardware stores, but can be constructed with othermaterials. The shape of the detector should be suchthat it does not compress or otherwise disturb floc-culent sediments. The detector head must be water-tight; no bare wires can be exposed to water. Thetubing can be filled with a tough, flexible epoxy orcomparable material, such as silicone RTV sealant tocover any bare wires inside. We used four-conductor

telephone cord (the type used to wire telephoneoutlets) to power the diode and carry the signal backto the amplifier. This wire is water-tight, fairly rugged,inexpensive, and readily available.

The output voltage of the transistor, which is highwhen light is blocked, is connected to the non-invertinginput of an op-amp (see Figure 3). This op-amp isconfigured as a voltage level detector. The user sets thetransistor output to a voltage level which will triggerthe buzzer when it exceeds the set point. The set pointcan be adjusted to calibrate the sensitivity of thedetector. The lower the voltage, the more sensitive thesensor is to light path interruption. The infrared diodelight source has a constant current supply (about10 mA) determined by Rd. The amplifier is a common714 op-amp, available at any electronics parts store.Pin numbers for the op-amp are shown on the sche-matic; pins 1 and 5 are not used. The circuit board usedin the assembly is a small perf-board, also availableat any electronics parts store. The entire circuitoperates on a 9-volt alkaline, transistor battery whichshould provide at least 24 h of continuous service. Toconserve battery power, the instrument can be turnedoff when not in use.

Using the detector

The detector we use in shallow waters is affixed to two2.5 cm diameter by 3.0 m long pieces of PVC pipe

Figure 1. Overview of several techniques and methods used to measure depth to the water/sediment interface in lakes with soft, unconsolidatedsediments.

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Figure 2. Drawing of the infrared-optical detector head. Crosshatch indicates sealant filler.

marked at 5 cm intervals. The detector is connectedwith epoxy, to a female 2.5 cm threaded PVC coupler,and the corresponding male coupler is glued to thepipe. The detector wire is threaded through the pipe,and then the pipe and detector are screwed together.Alternately, the detector can be attached to a weighted,metered cable. The only requirements are that thedepth detected (water depth) can be measured accuratelywith either a calibrated rod or calibrated line and thatthe detector wire is not unduly stressed during loweringand raising.

The detector is operated by lowering it toward thebottom, slowing the descent near the water/flocculentsediment interface. As the floc interface is penetrated,the buzzer will sound. At this point, the detector israised and lowered slowly with the pipe or line, and thedepths where the buzzer sounds and shuts off are noted.This range in depth, which can be measured veryprecisely if the water surface is calm, is typically about1 cm. The depth reading obtained is the distance fromthe water surface to the water/sediment interface.

The voltage output of the photo transistor wasmeasured over varying depths in the water column andat the water/sediment interface during testing at

shallow, hypereutrophic Newnans Lake, AlachuaCounty, Florida. The resulting profile can be dividedinto three zones as shown in Figure 4. Near the watersurface in Zone A, the output is very low, on the orderof 0.1 volt, and varies as a result of ambient light (thephoto transistor is sensitive to sunlight or artificiallight). Deeper in the water column in Zone B, wheresunlight is attenuated, the voltage stabilizes at about0.2–0.3 volts. The voltage increases rapidly to amaximum of about 3 volts in Zone C, when thedetector enters flocculent sediments. The user setpoint is established on the steep slope of this voltagechange. The buzzer will then sound when the voltageexceeds the set point, indicating the sediment surfaceis detected. The steepness of the slope of the line isproportional to the optical properties of the interface;i.e. the sharper the optical demarcation, the steeper theslope. This property of the electronic circuit allows thedetector to sense accurately the floc layer or the water/sediment interface, allowing the user to measure thedepth to the water/sediment interface. Attenuatedsignals from suspended particles in the water column,such as algal layers, will be too small to trigger falsealarms.

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Figure 3. Schematic diagram of electrical circuits and components for the infrared-optical detector.

In Lake Apopka, we measured depths to the sedi-ment surface at 46 stations using a weighted Secchi discand the IFD. We found that water depth measured witha Secchi disc averaged 10.4 ± 5.5 cm greater than waterdepth measured with our IFD. The depth sensed withthe IFD was less than that determined with the Secchidisc at 43 stations and no different at three stations.There are two reasons that the Secchi disc over-estimated depth. First, some of the stations weresampled when waves were 30 cm or more, undoubtedlyleading to overestimates with the Secchi disc. Underthese conditions, the sediment surface could be sensedmore precisely with the IFD than with the Secchi disc.Second, we relied on the IFD for our primary measure-ment. As a result Secchi disc measurements were takenless carefully than would have been the case if theywere the primary measure of water depth. Results,

however, show that the Secchi disc penetrated floc-culent sediments, resulting in an overestimate of waterdepth.

Conclusions

We have field tested the IFD in several lakes during twoyears of field work and have found it to be extremelyaccurate and reliable. Using the detector allowed us todetermine accurately the water/sediment interface withone measurement; the accuracy of data obtained withother techniques was much less and therefore lessreliable. This accurate measure of depth to the sedimentsurface also enabled us to consistently retrieve pistoncores with an intact mud/water interface and to maxi-mize the length of these cores.

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Acknowledgments

We acknowledge financial support from the St. JohnsRiver Water Management District, Palatka, Florida, andthe Carl S. Swisher endowment, University of FloridaFoundation. We acknowledge Mark Brenner for manyhelpful suggestions on the manuscript. We are gratefulto Phyllis Hansen, Jaye Cable, Andrew Chapman, andWilliam Kenney for assistance with the field work andtesting. We also wish to thank William Haller andMargaret Glen for their generous assistance in carrying

out the field program and Martha Love for beinghelpful in many ways.

References

Brenner, M. & M. W. Binford, 1988. Relationships betweenconcentrations of sedimentary variables and trophic state inFlorida lakes. Can. J. Fish. Aquat. Sci. 45: 294–300.

Schelske, C. L., 1997. Sediment and Phosphorus Deposition in LakeApopka. Final Report. Contract #96W213. Spec. Pub. SJ 97-SP21. St. Johns River Water Management District, Palatka,FL. 97 pp plus 6 appendices.

Figure 4. Profile of transistor output (volts) vs. depth (cm) in a field test at Newnans Lake, Florida. The profile is divided into three zones: A,B, and C. Zones A and B are in the water column, whereas zone C represents unconsolidated sediments. See text for additional explanation.

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