Exhibit 10.64


                        AMENDMENT NO. 4 TO LOAN AGREEMENT
                             DATED OCTOBER 25, 1996
               BY AND BETWEEN TANKNOLOGY-NDE INTERNATIONAL, INC.,
                           TANKNOLOGY/NDE CORPORATION,
                   TANKNOLOGY-NDE CONSTRUCTION SERVICES, INC.,
                      PROECO, INC. AND 2368692 CANADA, INC.
                                       AND
                              BANK ONE, TEXAS, N.A.

     This Amendment No. 4 ("Fourth Amendment") to the Loan Agreement dated as of
the 25th day of October,  1996, as amended (the "Loan Agreement"),  by and among
TANKNOLOGY-NDE   INTERNATIONAL,   INC.  (formerly  known  as  NDE  ENVIRONMENTAL
CORPORATION)  ("NDE"), a Delaware  corporation,  TANKNOLOGY/NDE  CORPORATION,  a
Delaware corporation, PROECO, INC., a Delaware corporation, 2368692 CANADA, INC.
(formerly  known  as  TANKNOLOGY   CANADA  (1988)  INC.),  a  Canadian   federal
corporation,   and  TANKNOLOGY-NDE   CONSTRUCTION  SERVICES,  INC.,  a  Delaware
corporation  (collectively,  "Borrower") and BANK ONE,  TEXAS,  N.A., a national
banking association (the "Bank") is entered into this 26th day of June 1998.

                              W I T N E S S E T H:

     WHEREAS,  Borrower and Bank entered into the Loan  Agreement on October 25,
1996,  as  amended  by the First  Amendment  dated  April 10,  1997,  the Second
Amendment dated May 20, 1997, and the Third Amendment dated December 23, 1997;

     WHEREAS,  Borrower  desires to increase the Revolving  Commitment under the
Loan Agreement;

     WHEREAS,  Borrower  desires to increase the advance  rate on the  Revolving
Commitment under the Loan Agreement;

     WHEREAS,  Borrower desires to include Borrower's Inventory in the Borrowing
Base calculation;

     WHEREAS,  Borrower desires to extend the Revolving  Commitment  Termination
Date of the Revolving Note;

     WHEREAS,  Bank is willing to agree to the foregoing in accordance with, and
subject to, the terms and conditions set forth herein;

     NOW, THEREFORE, in consideration of the promises herein contained, and each
intending to be legally bound hereby, the parties agree as follows:




                                       -1-

<PAGE>



     I.   Amendments to Loan Agreement.

     Any and all references to "NDE  ENVIRONMENTAL  CORPORATION"  as an existing
Borrower is hereby replaced with  "TANKNOLOGY-NDE  INTERNATIONAL,  INC." and any
and all references to "TANKNOLOGY CANADA (1988) INC." as an existing Borrower is
hereby replaced with "2368692 CANADA, INC."

     Article I, DEFINITIONS,  is amended by revising the following defined terms
(or their designated subparts) in their entirety to read as follows:

          "Accounts,"  "Chattel Paper," "Contracts,"  "Documents,"  "Equipment,"
          "Fixtures,"  "General   Intangibles,"   "Goods,"   "Instruments,"  and
          "Inventory", except as may be expressly provided otherwise in the Loan
          Agreement,  shall have the same  respective  meanings  as are given to
          those terms in the Uniform Commercial Code as presently adopted and in
          effect in the State of Texas.

          "Borrowing Base" means, at any time, the lesser of: (a) $9,000,000.00;
          or  (b)  the  amount   accurately   computed  on  the  Borrowing  Base
          Certificate  most recently  delivered to, and accepted by, the Bank in
          accordance  with Section 2.06 and other  relevant  provisions  of this
          Agreement,  and equal to eighty percent (80%) of Eligible  Accounts of
          the  Borrower,  plus the  lesser  of (i)  fifty  percent  (50%) of the
          then-current   market   value  of   Borrower's   Inventory,   or  (ii)
          $500,000.00.

          "Eligible  Account"  . . . (N) No more  than 25% of  account  debtor's
          aggregate   account  total  fails  to  satisfy   clause  (H)  of  this
          definition;

          "Maximum  Commitment Amount" means $9,000,000.00 as of the date of the
          Fourth Amendment.

          "Permitted Liens" . . . (I) Purchase money security  interests granted
          to secure not more than eighty  percent (80%) of the purchase price of
          assets, except as permitted by Sections 6.14 and 6.15, the purchase of
          which does not  violate  this  Agreement  or any  instrument  required
          hereunder;

          "Revolving Commitment Termination Date" means December 31, 2000.

          "Revolving  Note" means that certain  promissory  note in the original
          face  amount  of  $9,000,000.00  dated of even  date  with the  Fourth
          Amendment made by the Borrower payable to the order of the Bank in the
          form  attached as Exhibit "A" to the Fourth  Amendment,  together with
          all deferrals,  renewals,  extensions,  amendments,  modifications  or
          rearrangements  thereof,  which promissory note shall evidence certain
          advances to the  Borrower by the Bank  pursuant to Section 2.01 of the
          Loan Agreement.

          "Term Note Rate" means a per annum interest rate equal to the BANK ONE
          Base Rate in effect from time to time plus one percent (1.0%).


                                       -2-

<PAGE>




     Article  I,  DEFINITIONS,  is  further  amended  by  adding  the  following
definition:

          "Fourth  Amendment"  means  Amendment  No. 4 to this  Loan  Agreement,
          executed by Borrower and Bank on June 26, 1998.

     Article II, THE LOAN, of the Loan  Agreement is hereby  amended by revising
the following section in its entirety to read as follows:

     2.05 Payments of Term Loan Principal.  For the period  beginning on July 1,
     1998 until December 1, 1998 principal payments of the Loan evidenced by the
     Term Note  shall not be due,  except  as  provided  in  Section  2.08.  The
     principal  of the  Loan  evidenced  by the  Term  Note  will be  repaid  in
     thirty-six  (36) equal  consecutive  monthly  installments in the amount of
     $100,000.00 each, beginning on January 1, 1999, and continuing on the first
     day of each calendar  month  thereafter  until the Term Loan Maturity Date,
     when the entire unpaid  principal  amount then outstanding of the Term Note
     shall be due and payable.

     Section 5.21 of the Loan Agreement,  as amended by the Third Amendment,  is
hereby amended by revising that section in its entirety to read as follows:

     5.21 Debt Service Coverage Ratio. Maintain a Debt Service Coverage Ratio of
     not less than 1.20 to 1.0: (a) for the six (6) months ending June 30, 1998,
     it shall be calculated  on a six- month basis;  (b) for the nine (9) months
     ending  September 30, 1998,  it shall be calculated on a nine-month  basis;
     and (c) for the twelve (12) months ending December 31, 1998 and thereafter,
     it shall be calculated on a rolling four-quarter basis.

     Section  5.22  General and  Administrative  Expenses  is hereby  amended by
replacing  the reference  therein to "30%" with "25%" and is further  amended by
replacing the reference therein to "December 31, 1997" with "June 30, 1998".

     Section  6.01 Other  Indebtedness,  as amended by the Third  Amendment,  is
hereby  further  amended  by  replacing  the  reference  in  Subsection  (I)  to
"$750,000.00" with "$1,000,000.00".

     Section 6.11 Restricted  Payments,  as amended by the Third  Amendment,  is
hereby amended by moving the word "or" from immediately preceding Subsection (D)
of that section to  immediately  preceding the period at the end of that section
and is further amended by adding the following subsection thereto:

     "(E)  use  cash  consideration  for  the  purpose  of any  stock  or  asset
     acquisition  of or  from  any  Person  in an  amount  that  exceeds  in the
     aggregate $2,000,000.00"

     Section  6.14  Capital  Expenditures  Limitation,  as  amended by the Third
Amendment, is hereby further amended by replacing the clause therein which reads
"$2,500,000.00  in the 1998 fiscal year" with  "$3,000,000.00 in the 1998 fiscal
year"  and  by  replacing  the  clause   therein  which  reads  "not  more  than
$750,000.00" with "not more than $1,000,000.00".


                                       -3-

<PAGE>



     "Exhibit A-1," the form of Revolving  Note attached to the Loan  Agreement,
as amended by the Third  Amendment  and therein  referred to as "Exhibit  A", is
hereby replaced with Exhibit "A" attached to the Fourth Amendment.

     "Exhibit  B," the  form of  Compliance  Certificate  attached  to the  Loan
Agreement,  as amended by the Third  Amendment,  is hereby replaced with Exhibit
"B" attached to the Fourth Amendment.

     "Exhibit C," the form of Borrowing  Base  Certificate  attached to the Loan
Agreement,  as amended by the Third  Amendment,  is hereby replaced with Exhibit
"C" attached to the Fourth Amendment.

     "Exhibit  D-1" the form of Opinion of Counsel for Borrower  attached to the
Loan  Agreement,  as amended by the Third  Amendment and therein  referred to as
"Exhibit  G-1",  is hereby  replaced  with  Exhibit  "D"  attached to the Fourth
Amendment.

     "Schedule 1.01(a),  Collateral" attached to the Loan Agreement,  as amended
by the Second and Third  Amendments,  is hereby replaced with Schedule  1.01(a),
Collateral attached to this Fourth Amendment.

     "Schedule  3.10,  Collateral  Documents"  attached to the Loan Agreement is
hereby replaced with Schedule 3.10, Collateral Documents attached to this Fourth
Amendment.

     II. Conditions to the Effectiveness of the Fourth Amendment. As a condition
to the effectiveness of the Fourth Amendment by Bank, Borrower has satisfied the
following conditions:

          A. Receipt of Amended and Restated  Revolving Note,  Fourth  Amendment
     and Certificate of Compliance. The Bank shall have received the Amended and
     Restated  Revolving  Note (the form of which is attached  hereto as Exhibit
     "A"), multiple  counterparts of this Fourth Amendment,  as requested by the
     Bank and the Compliance  Certificate duly executed by an authorized officer
     for each Borrower (the form of which is attached hereto as Exhibit "B");

          B. Receipt of Certified Copy of Corporate  Proceedings and Certificate
     of  Incumbency.  The Bank shall have received from each Borrower  copies of
     all resolutions of its board of directors with respect to the  transactions
     set  forth  in this  Fourth  Amendment  and the  execution  of this  Fourth
     Amendment and the Amended and Restated  Revolving Note, such copy or copies
     to be  certified by the  Secretary or an Assistant  Secretary as being true
     and  correct  and in full  force  and  effect  as of the  date  hereof.  In
     addition,  the Bank shall have received from each Borrower a certificate of
     incumbency signed by the Secretary or an Assistant  Secretary setting forth
     (a) the names of the  officers  executing  this  Fourth  Amendment  and the
     Amended  and  Restated  Revolving  Note,  (b) the  office(s)  to which such
     Persons  have been  elected  and in which they  presently  serve and (c) an
     original specimen signature of each such person.

          C.  Receipt  of  the  Certified  Copy  of  Corporate  Proceedings  and
     Certificate of Incumbency.  Bank shall have received from DH Holdings Corp.
     ("DHH"), copies of all resolutions of their boards of directors with




                                       -4-

<PAGE>



     respect to the  transactions  contemplated by the this Fourth Amendment and
     the First Amendment to the Intercreditor and Subordination Agreement,  such
     copy or copies to be certified by the  Secretary or an Assistant  Secretary
     as being  true and  correct  and in full  force  and  effect as of the date
     hereof. In addition, the Bank shall have received from DHH a certificate of
     incumbency signed by the Secretary or an Assistant  Secretary setting forth
     (a)  the  names  of  the  officers   executing   First   Amendment  to  the
     Intercreditor and Subordination  Agreement, (b) the office(s) to which such
     Persons  have been  elected  and in which they  presently  serve and (c) an
     original specimen signature of each such person.

          D. Receipt of the First Amendment to Intercreditor  and  Subordination
     Agreement. Borrower, DHH and Bank shall have entered into a First Amendment
     to  Intercreditor  and  Subordination  Agreement  in the form  attached  as
     Exhibit "E".

          E.  Receipt of the Second  Amended and  Restated  Standby  Commitment.
     Bank,  Borrower  and  Proactive  Partners,  L.P.  ("Proactive")  shall have
     entered into a Second Amended and Restated  Standby  Commitment in the form
     attached as Exhibit "F".

          F.  Facility  Fee. As partial  consideration  for its agreement to the
     terms of the Fourth Amendment, Bank shall have received $40,000 prior to or
     contemporaneous with the execution of the Fourth Amendment.

          G.  Borrower's  Opinion of  Counsel.  Bank shall  have  received  from
     counsel for Borrower  written opinions in the form set forth on Exhibit "D"
     attached to the Fourth Amendment.

          H.   Satisfaction  of  Conditions  to  Fourth   Amendment.   Upon  the
     satisfaction  of  the  conditions  to  the  effectiveness  of  this  Fourth
     Amendment,  as set forth in this Article II hereof,  Borrower shall execute
     and  deliver  to Bank a  Certificate  of  Compliance,  and if Bank has been
     reasonably  satisfied that such conditions have been fulfilled,  Bank shall
     contemporaneously  provide a letter to Borrower and DHH stating  "Bank One,
     Texas, N.A. is satisfied that the conditions set forth in Article II of the
     Fourth  Amendment to that certain Loan  Agreement  dated  October 25, 1996,
     among the Bank and  Tanknology-NDE  International,  Inc.  et al.  have been
     fulfilled," whereupon this Fourth Amendment shall become effective.

     III.  Reaffirmation of  Representations  and Warranties.  To induce Bank to
enter into this Fourth  Amendment,  Borrower  hereby  reaffirms,  as of the date
hereof, its representations  and warranties  contained in Article IV of the Loan
Agreement and in all other documents executed pursuant thereto, and additionally
represents and warrants as follows:

          A.  The  execution  and  delivery  of this  Fourth  Amendment  and the
     performance by Borrower of its obligations  under this Fourth Amendment are
     within  the  Borrower's   corporate  power,  have  received  all  necessary
     governmental  approval (if any shall be required),  and do not and will not
     contravene  or  conflict  with  any  provision  of law or of any  agreement
     binding upon the Borrower.

                                       -5-

<PAGE>



          B. The Loan Agreement as amended by this Fourth  Amendment  represents
     the legal, valid and binding  obligations of Borrower,  enforceable against
     Borrower in  accordance  with its terms subject as to  enforcement  only to
     bankruptcy,  insolvency,  reorganization,  moratorium or other similar laws
     affecting  the  enforcement  of  creditors'  rights  generally  and general
     principles of equity.

          C. No Event of Default or Unmatured  Event of Default has occurred and
     is continuing as of the date hereof.

     IV. Defined  Terms.  Except as amended  hereby,  terms used herein that are
defined in the Loan Agreement shall have the same meanings herein.

     V.  Reaffirmation of Loan Agreement.  This Fourth Amendment shall be deemed
to be an amendment to the Loan  Agreement,  and the Loan  Agreement,  as amended
hereby,  is hereby  ratified,  approved and confirmed in each and every respect.
All  references  to  the  Loan  Agreement  herein  and in  any  other  document,
instrument,  agreement or writing shall hereafter be deemed to refer to the Loan
Agreement as amended hereby.

     VI. Entire Agreement.  The Loan Agreement, as hereby amended,  embodies the
entire agreement  between Borrower and Bank, and supersedes all prior proposals,
agreements and  understandings  relating to the subject matter hereof.  Borrower
certifies  that  it is  relying  on no  representation,  warranty,  covenant  or
agreement except for those set forth in the Loan Agreement as hereby amended and
the other documents previously executed or executed of even date herewith.

     VII.  Governing  Law.  THIS  FOURTH  AMENDMENT  SHALL  BE  GOVERNED  BY AND
CONSTRUED IN ACCORDANCE  WITH THE LAWS OF THE STATE OF TEXAS AND THE  APPLICABLE
LAWS OF THE UNITED  STATES OF AMERICA.  This Fourth  Amendment  has been entered
into in Harris County,  Texas,  and it shall be performable  for all purposes in
Harris County,  Texas.  Courts within the State of Texas shall have jurisdiction
over any and all disputes between  Borrower and Bank,  whether in law or equity,
including,  but not limited to, any and all disputes  arising out of or relating
to this  Fourth  Amendment  or any other  Loan  Document;  and venue in any such
dispute whether in federal or state court shall be laid in Harris County, Texas.

     VIII.  Severability.  Whenever  possible  each  provision  of  this  Fourth
Amendment shall be interpreted in such manner as to be effective and valid under
applicable  law,  but if  any  provision  of  this  Fourth  Amendment  shall  be
prohibited  by  or  invalid  under  applicable  law,  such  provision  shall  be
ineffective  to  the  extent  of  such   prohibition   or  invalidity,   without
invalidating the remainder of such provision or the remaining provisions of this
Fourth Amendment.

     IX. Execution in Counterparts. This Fourth Amendment may be executed in any
number of counterparts  and by the different  parties on separate  counterparts,
and each  such  counterpart  shall be  deemed  to be an  original,  but all such
counterparts shall together constitute but one and the same agreement.

     X. Section Captions. Section captions used in this Fourth Amendment are for
convenience  of reference  only, and shall not affect the  construction  of this
Fourth Amendment.

                                       -6-

<PAGE>



     XI. Successors and Assigns. This Fourth Amendment shall be binding upon the
Borrower and Bank and their respective  successors and assigns,  and shall inure
to the benefit of the  Borrower  and Bank,  and the  respective  successors  and
assigns of Bank.

     XII. Non-Application of Chapter 15 of Texas Credit Codes. The provisions of
Chapter 15 of the Texas  Credit Code  (Vernon's  Texas Civil  Statutes,  Article
5069-15) are specifically declared by the parties hereto not to be applicable to
the Loan  Agreement as hereby  amended or any of the other Loan  Documents or to
the transactions contemplated hereby.

     XIII.  Notice.  THIS FOURTH AMENDMENT  TOGETHER WITH THE LOAN AGREEMENT AND
THE OTHER LOAN DOCUMENTS  REPRESENT THE FINAL AGREEMENT  BETWEEN THE PARTIES AND
MAY NOT BE CONTRADICTED BY EVIDENCE OF PRIOR, CONTEMPORANEOUS OR SUBSEQUENT ORAL
AGREEMENTS OF THE PARTIES.  THERE ARE NO UNWRITTEN ORAL  AGREEMENTS  BETWEEN THE
PARTIES.


                                       -7-

<PAGE>



     IN WITNESS WHEREOF, the parties hereto have caused this Fourth Amendment to
be duly executed as of the day and year first above written.


                                          BORROWER:

                                          TANKNOLOGY-NDE INTERNATIONAL, INC.
                                          (formerly known as NDE ENVIRONMENTAL
                                          CORPORATION)

                                          By:    //s// JAY ALLEN CHAFFEE
                                               -------------------------------
                                                   Jay Allen Chaffee
                              Chairman of the Board

                                          TANKNOLOGY/NDE CORPORATION

                                          By:    //s// JAY ALLEN CHAFFEE
                                               -------------------------------
                                                   Jay Allen Chaffee
                              Chairman of the Board

                                          TANKNOLOGY-NDE CONSTRUCTION
                                          SERVICES, INC.

                                          By:    //s// JAY ALLEN CHAFFEE
                                               -------------------------------
                                                   Jay Allen Chaffee
                              Chairman of the Board

                                          PROECO, INC.

                                          By:    //s// JAY ALLEN CHAFFEE
                                               -------------------------------
                                                   Jay Allen Chaffee
                              Chairman of the Board

                                          2368692 CANADA, INC.(formerly known as
                                          TANKNOLOGY CANADA (1988) INC.)

                                          By:    //s// JAY ALLEN CHAFFEE
                                               -------------------------------
                                                   Jay Allen Chaffee
                                                   President








                                       -8-

<PAGE>



                                          BANK:

                                          BANK ONE, TEXAS, N.A.


                                          By:    //s// Charles Kingswell-Smith
                                               -------------------------------
                             Charles Kingswell-Smith
                              Senior Vice President


                                       -9-

<PAGE>



                                SCHEDULE 1.01(a)

                                   COLLATERAL

Tanknology-NDE International, Inc.
         (a)        Pledged  Stock of  Tanknology/NDE  Corporation,  a  Delaware
                    corporation,  Tanknology-NDE  Construction Services, Inc., a
                    Delaware corporation,  ProEco, Inc., a Delaware corporation,
                    and 2368692 Canada, Inc., a Canadian federal corporation
          (b)       Bank  Accounts  and  Pledged  Certificates  of  Deposit
          (c)       Inventory

Tanknology/NDE Corporation
         (a)        Bank Accounts
         (b)        Accounts Receivable
         (c)        Vehicles
         (d)        Diagnostic Equipment (including equipment on Vehicles)
         (e)        Inventory
         (f)        Furniture and Office Equipment
         (g)        Patents
         (h)        Trademarks
         (i)        Proprietary Software
         (j)        Licensing Agreements

ProEco, Inc.
         (a)        Accounts Receivable
         (b)        Bank Accounts
         (c)        Furniture and Office Equipment
         (d)        Licensing Agreements
         (e)        Inventory

Tanknology-NDE Construction Services, Inc.
         (a)        Accounts Receivable
         (b)        Bank Accounts
         (c)        Patents
         (d)        Furniture and Office Equipment
         (e)        Licensing Agreements
         (f)        Inventory


                                    1.01(a)-1

<PAGE>



                                  SCHEDULE 3.10

                              COLLATERAL DOCUMENTS

1.       Stock Powers executed by Tanknology-NDE International, Inc.
         a.       Tanknology/NDE Corporation dated October 25, 1996
         b.       ProEco, Inc. dated October 25, 1996
         c.       2368692 Canada, Inc. dated October 25, 1996
         d.       Tanknology-NDE  Construction  Services,  Inc. dated December
                  23, 1997

2.       Security Agreements
         a.       Tanknology-NDE International, Inc. and Bank One, Texas dated
                  October 25, 1996
         b.       Tanknology/NDE  Corporation  and Bank One, Texas dated October
                  25, 1996
         c.       ProEco, Inc. and Bank One, Texas dated October 25, 1996
         d.       Tanknology-NDE  Construction  Services,  Inc. dated December
                  23, 1997

3.       Financing Statements

         Tanknology-NDE International, Inc.
         CALIFORNIA: Secretary of State
         DELAWARE: Secretary of State
         TEXAS: Secretary of State & Travis County

         ProEco, Inc.
         DELAWARE: Secretary of State
         TEXAS: Secretary of State & Travis County

         Tanknology/NDE Corporation
         CALIFORNIA: Secretary of State
         DELAWARE: Secretary of State
         GEORGIA: Gwinnett County
         ILLINOIS: Secretary of State
         MASSACHUSETTS: Secretary of Commonwealth & Agawam Township
         MISSOURI: Secretary of State
         NEW JERSEY: Secretary of State
         OHIO: Secretary of State & Fairfield/Franklin Counties
         TEXAS: Secretary of State & Travis County

         Tanknology-NDE Construction Services, Inc.
         DELAWARE: Secretary of State
         TEXAS: Secretary of State

4.       Certificate of Title Liens
                  Tanknology/NDE Corporation



                                     3.10-1

<PAGE>



5.       Assignments of Patents
                  ProEco, Inc.
                  Tanknology/NDE Corporation

6.       Assignments of Trademarks
                  Tanknology/NDE Corporation


                                     3.10-2

<PAGE>




                                   EXHIBIT "A"

                       AMENDED AND RESTATED REVOLVING NOTE

$9,000,000.00                                                      June 26, 1998

     FOR  VALUE  RECEIVED,   TANKNOLOGY-NDE  INTERNATIONAL,   INC.,  a  Delaware
corporation, TANKNOLOGY/NDE CORPORATION, a Delaware corporation, PROECO, INC., a
Delaware corporation,  TANKNOLOGY-NDE  CONSTRUCTION  SERVICES,  INC., a Delaware
corporation,  and 2368692 CANADA, INC., a Canadian federal  corporation,  all of
the foregoing having an address at 8900 Shoal Creek,  Bldg. 200,  Austin,  Texas
78757, (collectively, "Borrower") unconditionally promise to pay to the order of
BANK ONE, TEXAS, NATIONAL ASSOCIATION, (herein called "Bank"), at its offices at
910 Travis,  Houston,  Texas 77002,  the principal  sum of NINE MILLION  DOLLARS
($9,000,000.00)  or,  if less,  the  aggregate  unpaid  principal  amount of all
Revolving  Loans  (as  defined  in the Loan  Agreement)  made by the Bank to the
Borrower  pursuant to the Loan  Agreement,  as shown in the records of the Bank,
outstanding on such date.

     The undersigned also promise to pay interest on the unpaid principal amount
hereof  from  time to time  outstanding  from the  date  hereof  until  maturity
(whether by acceleration  or otherwise) and, after maturity,  until paid, at the
rates  per annum and on the dates  specified  in the Loan  Agreement;  provided,
however,  that in no event  shall  such  interest  exceed the  Maximum  Rate (as
hereinafter defined).

     "Maximum Rate" means the Maximum Rate of  non-usurious  interest  permitted
from day to day by Applicable Law.

     "Applicable  Law" means that law in effect from time to time and applicable
to this Note which  lawfully  permits the charging and collection of the highest
permissible lawful,  non-usurious rate of interest on this Note,  including laws
of the State of Texas and laws of the United  States of America.  It is intended
that Article 1.04,  Title 79, Revised Civil Statutes of Texas,  1927, as amended
(Article 5069-1.04, as amended, Vernon's Texas Civil Statutes) shall be included
in the laws of the State of Texas in  determining  Applicable  Law;  and for the
purpose of  applying  said  Article  1.04 to this  Note,  the  interest  ceiling
applicable  to this Note under said Article 1.04 shall be the  indicated  weekly
rate ceiling from time to time in effect. The Borrower and the Bank hereby agree
that Chapter 15 of Subtitle 3, Title 79, Revised Civil Statutes of Texas,  1925,
as amended,  shall not apply to this Note or the loan transaction  evidenced by,
and  referenced  in, the Loan  Agreement  (hereinafter  defined)  in any manner,
including without limitation, to any account or arrangement evidenced or created
by, or provided for in, this Note.

     In no event shall the  aggregate  of the  interest  on this Note,  plus any
other  amounts  paid in  connection  with the loan  evidenced by this Note which
would under Applicable Law be deemed  "interest," ever exceed the maximum amount
of interest which, under Applicable Law, could be lawfully charged on this Note.

                                                                    ------------
                                                                     Initial for
                                                                  Identification
                                       A-1

<PAGE>



The Bank and the Borrower  specifically  intend and agree to limit contractually
the interest  payable on this Note to not more than an amount  determined at the
Maximum Rate. Therefore, none of the terms of this Note or any other instruments
pertaining to or securing this Note shall ever be construed to create a contract
to pay  interest  at a rate in excess  of the  Maximum  Rate,  and  neither  the
Borrower nor any other party liable herefor shall ever be liable for interest in
excess of that  determined  at the  Maximum  Rate,  and the  provisions  of this
paragraph  shall  control  over  all  provisions  of this  Note or of any  other
instruments pertaining to or securing this Note. If any amount of interest taken
or  received  by the Bank shall be in excess of the  maximum  amount of interest
which,  under  Applicable  Law, could lawfully have been collected on this Note,
then the excess shall be deemed to have been the result of a mathematical  error
by the  parties  hereto and shall be  refunded  promptly  to the  Borrower.  All
amounts paid or agreed to be paid in connection with the indebtedness  evidenced
by this Note which would under Applicable Law be deemed "interest" shall, to the
extent permitted by Applicable Law, be amortized, prorated, allocated and spread
throughout the full term of this Note.

     This Note is the  Revolving  Note  referred  to in and is  entitled  to the
benefits of a certain Loan  Agreement,  dated as of October 25, 1996, as amended
by that First Amendment  dated April 10, 1997,  that Second  Amendment dated May
20, 1997, that Third Amendment dated December 23, 1997 and that Fourth Amendment
dated of even  date  herewith  (as the same may be  further  amended,  modified,
supplemented,  extended, rearranged and/or restated from time to time, the "Loan
Agreement"),  entered  into by and  among  Tanknology-NDE  International,  Inc.,
(f/k/a NDE Environmental  Corporation) et al., as Borrower, and Bank One, Texas,
National  Association  and secured by the Collateral  Documents (as such term is
defined in the Loan  Agreement).  Reference is hereby made to the Loan Agreement
for a statement of the  prepayment  rights and penalties and  obligations of the
Borrower,  a description of the properties and assets mortgaged,  encumbered and
assigned, the nature and extent of the security and the rights of the parties to
the Collateral Documents in respect of such security, and for a statement of the
terms and conditions  under which the due date of this Note may be  accelerated.
Upon the occurrence of any Event of Default as specified in the Loan  Agreement,
the principal  balance hereof and the interest accrued hereon may be declared to
be forthwith  due and payable in  accordance  with the Loan  Agreement,  and any
indebtedness  of the  holder  hereof to the  Borrower  may be  appropriated  and
applied hereon.

     In addition to and not in  limitation  of the  foregoing,  the  undersigned
further agrees, subject only to any limitation imposed by applicable law, to pay
all  reasonable  expenses,   including  reasonable  attorneys'  fees  and  legal
expenses,  incurred  by the holder of this Note in  endeavoring  to collect  any
amounts payable  hereunder which are not paid when due,  whether by acceleration
or otherwise.

     All parties hereto, whether as makers,  endorsees, or otherwise,  severally
waive presentment for payment,  demand, protest, notice of intent to accelerate,
notice of acceleration and notice of dishonor.

     This Note is issued in substitution for, and in replacement,  modification,
rearrangement,  renewal  and  extension  of, but not in  extinguishment  of, the
outstanding principal indebtedness evidenced by that certain note of


                                                                    ------------
                                                                     Initial for
                                                                  Identification
                                       A-2

<PAGE>



Tanknology-NDE  International,   Inc.  (f/k/a  NDE  Environmental  Corporation),
Tanknology/NDE   Corporation,   ProEco,   Inc.,  2368692  Canada,  Inc.  (f/k/a/
Tanknology Canada (1988) Inc.) and Tanknology-NDE  Construction Services,  Inc.,
dated December 23, 1997,  payable to the order of Bank One,  Texas,  N.A. in the
original   principal  sum  of  $5,000,000.00,   (the  "Prior  Note");  it  being
acknowledged and agreed by Borrower that the indebtedness evidenced by this Note
constitutes an extension and renewal of the outstanding  principal  indebtedness
evidenced  by the Prior Note,  and that all security  interests  and other liens
which  secure  the  repayment  of the Prior Note  shall  continue  to secure the
indebtedness evidenced by this Note.

     THIS NOTE SHALL BE DEEMED TO BE A CONTRACT MADE UNDER THE LAWS OF THE STATE
OF TEXAS  AND  CONSTRUED  IN  ACCORDANCE  WITH THE LAWS OF THE  STATE OF  TEXAS,
WITHOUT REGARD TO PRINCIPLES OF CONFLICTS OF LAW.

     EXECUTED this 26th day of June, 1998.

TANKNOLOGY-NDE INTERNATIONAL,
INC.

By:    //s// JAY ALLEN CHAFFEE
    -------------------------------
         Jay Allen Chaffee
         Chairman of the Board

TANKNOLOGY/NDE CORPORATION


By:    //s// JAY ALLEN CHAFFEE
    -------------------------------
         Jay Allen Chaffee
         Chairman of the Board

TANKNOLOGY-NDE CONSTRUCTION
SERVICES, INC.


By:    //s// JAY ALLEN CHAFFEE
    -------------------------------
         Jay Allen Chaffee
         Chairman of the Board
PROECO, INC.


By:    //s// JAY ALLEN CHAFFEE
    -------------------------------
         Jay Allen Chaffee
         Chairman of the Board

2368692 CANADA, INC.


By:    //s// JAY ALLEN CHAFFEE
    -------------------------------
          Jay Allen Chaffee
          President


                                       A-3

<PAGE>



                                   EXHIBIT "B"


                             Compliance Certificate

     I, the  ____________________________ of TANKNOLOGY-NDE INTERNATIONAL,  INC.
(the  "Company"),  pursuant to Section  5.05 of the Loan  Agreement  dated as of
October 25, 1996, as amended by the First  Amendment  dated April 10, 1997,  the
Second  Amendment  dated May 20, 1997,  the Third  Amendment  dated December 23,
1997,  and the  Fourth  Amendment  dated June 26,  1998,  by and among BANK ONE,
TEXAS, N.A. ("Bank") and the Company et al. (the "Agreement") do hereby certify,
as of the date hereof, that to my knowledge:

         1.         No  Event of  Default  (as  defined  in the  Agreement)  has
                    occurred  and is  continuing,  and  no  Unmatured  Event  of
                    Default (as defined in the  Agreement)  has  occurred and is
                    continuing  except for the following events (include actions
                    taken to cure such situations):
                    ------------------------------------------------------------
                    ------------------------------------------------------------
                    ------------------------------------------------------------
                    ------------------------------------------------------------
                    ------------------------------------------------------------
                    -----------------------------;

         2.         No material  adverse  change has occurred in the  condition,
                    financial or otherwise, of the Company since
                    ----------------;

         3.         Except as otherwise stated in the Schedule, if any, attached
                    hereto,  each of the  representations  and warranties of the
                    Company contained in Article IV of the Agreement is true and
                    correct in all respects; and


                                       B-1

<PAGE>



         4.         The  Company's  financial  condition  for the  month  ending
                    __________ is as follows:

<TABLE>
<CAPTION>
       Financial Covenant          Time Period           Required Ratio                                           Actual Ratio
<S>                               <C>             <C>                                                            <C>
================================= ==============  =============================================================  ==============
(a) Net Worth                      Term of Loan    Not less than the  Consolidated  Net  Worth as of  12/31/97,
                                                   with allowable  cumulative interim losses during fiscal year
                                                   1998 of not more than $1,000,000.00 (tested as of the end of
                                                   each calendar quarter),  and as of 12/31/98, and thereafter,
                                                   maintain  at least  the  Consolidated  Net  Worth in  effect
                                                   12/31/97  plus 70% of  Borrower's  Net Income (if  positive)
                                                   after 12/31/98.

(b) Capital Expenditures           Term of Loan    Not more than  $3,000,000 for fiscal 1998 and $2,000,000 for
                                                   each year thereafter.

(c) Debt Service Coverage Ratio    Term of Loan    Not less than 1.2 to 1.0

(d) Adjusted  Liabilities  to      Term of Loan    12/23/97 - 6/30/98 not more than 2.25 to 1.0; 7/1/98 - 
    Adjusted Net Worth                             9/30/98  not more  than 2.00 to 1.0;  10/1/98 - 6/30/99  not
                                                   more than 1.75 to 1.0;  7/1/99 - 12/31/99 not more than 1.25
                                                   to 1.0; and after 12/31/99 not more than 1.0 to 1.0.

================================= ==============  =============================================================  ==============
</TABLE>


This certificate is executed this ___ day of ___________ 199__.


                       TANKNOLOGY-NDE INTERNATIONAL, INC.


                                            -----------------------------------
                       By: ______________________________
                       Its: ______________________________



                                       B-2

<PAGE>



                                   EXHIBIT "C"

                           BORROWING BASE CERTIFICATE

     I, the  ___________________________  of TANKNOLOGY-NDE INTERNATIONAL,  INC.
(the  "Company"),  pursuant to Section  2.06 of the Loan  Agreement  dated as of
October 25, 1996, as amended by the First  Amendment  dated April 10, 1997,  the
Second  Amendment  dated May 20, 1997,  the Third  Amendment  dated December 23,
1997, and the Fourth Amendment dated as of June 26, 1998, by and among BANK ONE,
TEXAS, N.A. ("Bank") and the Company et al. (the "Agreement") do hereby certify,
as of the date hereof, that to my knowledge:

     1. The Borrowing Base calculated  pursuant to the Agreement effective as of
____________, 199__, is $___________.

     2. The  Borrowing  Base  stated in  paragraph 1 hereof is the lesser of (a)
$9,000,000.00, or (b) eighty percent (80%) of Eligible Accounts of the Borrower,
plus the lesser of (i) fifty percent (50%) of the  then-current  market value of
Borrower's Inventory, or (ii) $500,000.00.

     3.  The  Eligible  Accounts  of the  Borrower  total  $_________,  which is
comprised of the following components:

          a.   Total of Accounts meeting the following criteria: $___________

               (1) The Account  arose from a bona fide outright sale of Goods by
          the Borrower or from bona fide services performed by the Borrower, and
          such Goods have been  shipped to the  appropriate  account  debtors or
          their designees (or the sale has otherwise been  consummated),  or the
          services have been performed for the appropriate account debtors;

               (2) The Account is based upon an  enforceable  order or contract,
          written or oral, for Goods shipped or held or for services  performed,
          and the same were shipped,  held, or performed in accordance with such
          order or contract;

               (3) The title of the  Borrower to the Account  and,  except as to
          the account debtor, to any Goods is absolute and is not subject to any
          prior assignment,  claim, lien, or security interest, except Permitted
          Liens;

               (4) The  amount  shown on the  books of the  Borrower  and on any
          invoice or statement  delivered to the Bank is owing to the  Borrower;
          and

               (5) The account debtor has not returned or refused to retain,  or
          otherwise notified the Borrower of any dispute concerning,  or claimed
          nonconformity  of, any of the Goods or services from the sale of which
          the Account arose;


                                       C-1

<PAGE>



                  b. Minus the sum of:

               (1)  $____________  for any  claim  of  reduction,  counterclaim,
          set-off,   recoupment,  or  any  claim  for  credits,  allowances,  or
          adjustments  by the account  debtor of any of the foregoing  Accounts,
          because of  returned,  inferior,  or damaged  Goods or  unsatisfactory
          services, or for any other reason;

               (2)  $____________  for any partial payment that has been made on
          any of the foregoing Accounts by anyone;

               (3) $____________  for any part of any of the foregoing  Accounts
          that is due and  payable  more than  thirty (30) days from the date of
          the invoice therefor;

               (4) $____________  for any part of any of the foregoing  Accounts
          is more than ninety (90) days past due or is outstanding more than one
          hundred twenty (120) days from the date of the invoice therefore;

               (5)  $____________  for any of the foregoing  Accounts that arise
          out of a contract  with, or order from, an account debtor that, by its
          terms,  forbids or makes void or  unenforceable  the assignment by the
          Borrower to the Bank of the Account arising with respect thereto;

               (6)  $____________  for the  amount or value of any  note,  trade
          acceptance,  draft, or other Instrument with respect to, or in payment
          of, any of the foregoing  Accounts,  or any Chattel Paper with respect
          to the Goods  giving  rise to any such  Account,  unless,  if any such
          Instrument  or  Chattel  Paper has been  received,  the  Borrower  has
          notified  the Bank  and,  at the  Banks's  request,  has  endorsed  or
          assigned and delivered the same to the Bank;

               (7) $____________ for any Account payable by an account debtor as
          to which the  Borrower  has  received  any  notice of the death of the
          account  debtor  or  a  partner   thereof;   or  of  the  dissolution,
          termination of existence, insolvency, business failure, appointment of
          a receiver for any part of the property of, assignment for the benefit
          of  creditors  by, or the filing of a petition  in  bankruptcy  or the
          commencement of any proceeding under any bankruptcy or insolvency laws
          by or against, the account debtor;

               (8)  $____________  for any Account  payable by an account debtor
          that is a Subsidiary or other Affiliate of the Company;

               (9)  $____________  for any Account  payable by an account debtor
          whose  principal  place of business is outside of the United States of
          America, its territories and possessions;

               (10)  $____________ for the aggregate amount of all Accounts owed
          by any  account  debtor  as to which  more  than  25% of such  account
          debtor's aggregate

                                       C-2

<PAGE>



                  Account  total is more  than  ninety  (90) days past due or is
                  outstanding  more than one hundred  twenty (120) days from the
                  date of the invoice therefore;

                           (11)  $____________  for any Account  insofar as such
                  Account constitutes more than twenty-five percent (25%) of the
                  aggregate total of Eligible Accounts in (a) hereinabove; and

                           (12)  $____________  for any  Account  that  Bank has
                  given  notice to Company that the Bank has deemed such Account
                  ineligible (i) because of a reasonable  uncertainty  about the
                  creditworthiness  of the account  debtor;  or (ii) because the
                  Bank  otherwise  reasonably  considers  the  collateral  value
                  thereof to the Bank to be  impaired  or its ability to realize
                  such value to be insecure.

     4. Borrower's Inventory totals $______________.

     5. The  worksheet  used to  calculate  the  Borrowing  Base is  attached as
Exhibit "A".

     Capitalized  terms used  herein  shall have the  meanings  assigned  in the
Agreement unless otherwise defined herein.

     This certificate is executed this ___ day of ____________ 199__.

                       TANKNOLOGY-NDE INTERNATIONAL, INC.


                        By: _____________________________
                       Its: _____________________________


                                       C-3

<PAGE>



                                   Exhibit "A"
                          to Borrowing Base Certificate

                                    Worksheet

                                       C-4

<PAGE>



                                                                     EXHIBIT "D"

                     Form of Opinion of Counsel for Borrower
                              Baker & Botts, L.L.P.

     (1) The Borrower and the  Subsidiaries  are  corporations  duly  organized,
existing,  and in good  standing  under the Laws of their  respective  states of
incorporation  [naming such states] and are  qualified to transact  business and
are in good  standing in those  states  where the nature of business or property
owned by them requires  qualification,  as set forth in Schedule 4.01,  attached
hereto and made a part hereof,  and, to the knowledge of such  counsel,  are not
required to be qualified as a foreign corporation in any other jurisdiction;

     (2) The  Borrower  has  the  power  to  execute  and  deliver  this  Fourth
Amendment,   to  borrow  money  hereunder,  to  grant  the  Collateral  required
hereunder,  to execute and deliver the Amended and Restated  Revolving Note, and
Collateral Documents, and to perform its obligations hereunder and thereunder;

     (3) All corporate actions by the Borrower and all consents and approvals of
any Persons necessary to the validity of this Fourth Amendment,  the Amended and
Restated Revolving Note, the Collateral Documents, and each other document to be
delivered hereunder have been duly taken or obtained, and this Fourth Amendment,
Amended and Restated  Revolving  Note,  and the Collateral  Documents,  and such
other  documents do not conflict with any provision of the charter or by-laws of
the Borrower,  or of any  applicable  Laws, or any other  agreement  binding the
Borrower or its property of which,  after reasonable  inquiry,  such counsel has
knowledge; and

     (4) This Fourth  Amendment,  the Amended and Restated  Revolving  Note, and
Collateral  Documents to be delivered  hereunder have been duly executed by, and
each is a valid and binding  obligation of, the Borrower;  each of the foregoing
documents is in all respects sufficient to achieve its purported function and is
enforceable  in  accordance  with its terms,  except as  limited by  bankruptcy,
insolvency,   reorganization,   moratorium,  or  other  similar  laws  affecting
creditors' rights generally or by general equitable principles.

                                       D-1

<PAGE>



                                   EXHIBIT "E"

          First Amendment to Intercreditor and Subordination Agreement

                                       E-1

<PAGE>


                                   EXHIBIT "F"

                 Second Amended and Restated Standby Commitment


                                       F-1